Haven't posted here in quite awhile though have used trip advisor many times. Usually very helpful. This post is to give back a bit.
For some reason, we had a hard time figuring out the best way to do a day trip to Versailles today despite going through lots of posts on Trip Advisor and elsewhere. So many ask the same question about the train/best way to get from Paris, etc.
Context: this will hopefully be helpful for those who a) want to do Versailles in one day, don't have small children and who enjoy at least walking if not a bit more.
Getting to Versailles:
We took the RER C line as many here suggested. We'd been told locally to take it from Saint Michele metro station (which can be done) which wasn't necessary. Since we were staying in the 7th, it was easy to just get the RER at any station on the line, in our case Invalides. 2.95/person. Train leaves every half hour. Wonderfully convenient.
Many posts here urge people to buy tickets in advance and we agree. But, we had trouble with the Versailles website (maybe start of high season tech issues) so asked around and, sure enough, we were able to get them at FNAC, a local music/video/general merchandise store that has a travel agency and separate "Spectacles" area where you can buy Versailles tix. There are at least 5 ticket options but, for a half day or full day trip, the 25 Euro "passport" is the way to go since it gives you unfettered access to everything you'll want to see there.
Arriving at Versailles:
Versailles Rive Gauche (as distinct from other stops with Versailles in their name and as advised by many others) is the place to get off closest to the "Chateau" (or palace or 'main palace' as different people call it). It's just a 10 minute walk to the Palace. Exit train station, see Starbucks just across the way. Make right and walk about 5 minutes up to an obvious left turn onto the big road that goes right into Versailes Palace. If you go left from the train station, you can head into Versailles town, good place to buy picnic supplies or other necessities. We didn't do that. You can't miss the main entrance to the palace as it's marked by a huge statue of Louis XIV on a horse. As soon as you pass the horse, you realize that all the post on this board urging to buy advance tix were right. If you don't, there's a long line to wait and buy them. So don't put yourself in that boat. However, one place where we'd differ from many others: it wasn't at all necessary to get there at opening time (9am for high season starting April 1). We'd intended to but it didn't work that way. We ended up arriving at around 11am and that worked fine for a relaxing but pretty through visit that lasted until 4pm. When you come in, you have two choices for entry. Either follow the signs for "A" which is for ticket holders and be prepared to check any pack or large bag. This entry is to enter and tour the big palace (aka "Chateaux"). We didn't do that first because the much-talked about fountain show started at 11 (only done twice on weekend days) so we took entry option #2, which is to the left of where the "A" entry is. You go under a stone arch and will be asked for a ticket before being able to go further. The same passport ticket is good for repeated entry everywhere so you just show, go and hold on to the stub.
Pretty cool but, to be honest, didn't wow us. I think since we'd never been to Versailles before we expected 500 foot high streams of water and a live orchestra. The fountains themselves (behind the palace and ranging out into the gardens) are numerous, beautiful and interesting. But the music is piped in on speakers hidden behind trees. That said, not a worry and it was nice. We didn't stay until the fountains stopped shooting at noon; no need to.
This was one topic we couldn't figure out specifically from all the posts here. So here's the deal.
If you can and even somewhat enjoy riding a bike, plan for that. Have your stuff in a daypack so you can do it. As others have posted, you can rent bikes either in Versailles town (that left turn from the train station mentioned above) or within the palace complex. We knew there was also a tour company (Fat Tire) that some people use but didn't consider that. To our way of thinking, there's no reason to rent in town or use a tour company. After gawking at the fountains and gardens/forests before us...and the really impressively large and long "Grand Canal" that's like the Washington DC mall reflecting pool times ten, we made our way down stairs to another gate, beyond which bikes were allowed as was evident from all the riders we could see. Off to the right is a spot called "Petit Venise" which can't be missed. It's bustling and has a restaurant and boat rental. The boat rental is for rowboats that can be used on the great canal and looked very fun but we didn't do that. We did rent bikes here. 15 euros/bike for half a day and includes a lock. You won't confuse these bikes with those in the Tour or on the slops in Marin County, California but they are totally fine and adequate for this purpose: touring Versailles. We set off and ended up using the bikes to easily cover much more ground than otherwise would have been possible--this is why we'd recommend them for those that can/like to bike. There is also a tourist train, segways and golf carts for those so inclined.
If ever there was a great place to do a picnic it's at the far end of the great canal on a day in late April with blue sunny skies like today. We'd picked up the requisite cheese (some Cantal, Epoisses and aged Gouda), artisan sausage, Poilane bread, fresh fruits (including some amazing pears and raspberries we just don't get at hom in the US), a small box of those addictive macaroons from La Duree on the Champs Elysee and a few chocolates from one of the many small, artisan and generally amazing cholatiers all over Paris. Oh, and of course, a bottle of good Burgundy. Alll this and a couple of plates, cups, blanket, etc. were easy to carry in two day packs on our backs while riding. Had a freakin' delightful picnic on the grass on the perfect day.
Non Palace Spots to Hit
We'd read different reviews of the secondary sights: the Grand Trianon and Petit Trianon but did both (again, much easier on a bike). The GT is pretty grand but doesn't do a lot that the main Palace won't. Guess this depends on your interest to go especially deep in the history of the period. We went through it quickly. The Petit Trianon is pretty cool simply because it is relative small and was Marie Antoinette's hideaway. The place exudes Marie Antoinette. Definitely worthwhile.
Also popular are the vast gardens. We didn't explore these as much as we might have but, it seemed to us like they were mostly trees, shrubs and hedges without many (and?) flowers. Maybe we missed those. Either way, the gardens are pretty amazing also and you get some sense for them riding around on a bike.
The Main Palace ("Chateaux")
Really as advertised--it's amazing on many levels. We especially enjoyed the King and Queen bedrooms, ante chambers and, especially, the Hall of Mirrors where WWI ended with the treaty of Versailles. There's also a photography exhibit underway now which we found fascinating: photos from 1850 until the present that feature the gardens, all the buildings and architecture and all the significant events and vistors. Shots included Leonid Brezhnev, JFK and Jackie, Reagan and Thatcher, etc.
Walking back to the train and getting back to Paris was as easy as going. The trains leave frequently. One good tip on your way back: all trains leaving Versailles Rive Gauche go through Paris so don't worry about checking the deparature board. Just board any train to get back into the city. Easy as pie.
Hope this is helpful for at least a few of you. We definitely recommend doing this for half a day or so if in the Paris area for at least 3 or 4 days. And, especially this time of year.