We happen to be doing this exact itinerary now! Just finishing up the trip. We visited Arenal (La Fortuna), Monteverde and are now in Manuel Antonio. We will have stayed 3 nights in each. We're in our mid-30s and I happen to be pregnant so we haven't been able to do too much of the adventurous stuff Costa Rica has to offer and our trip sounds very similar to yours.
ARENAL: I wouldn't skip Arenal. It's true the volcano can be hit or miss (can't see the very top of it this time of year) but the area is beautiful and the hot springs are great. We stayed at the Volcano Lodge and it was very nice - felt very resort like but I got a great price (about $80/night). This price included breakfast and the breakfast was really good... coffee, juices, great selection of fruit, sausage, cooked to order eggs, pancakes etc. The rooms are all single story (no one above you) and ours had a little porch with rocking chairs and a view of the volcano. At night you could hear a symphony of forest sounds. The weather was warm with a little rain and some humidity but not too much.
As for hot springs - I'm sure Tabacon is a very nice hot springs place (at $85/person per day) but we LOVED The Springs Resort which currently has a deal for $50 for a 2 day pass. It's a little tough to get to (bumpy road to get there) but so worth it. I think there were about 18 mineral hot spring pools in the most lush tropical environment. We actually spent a whole day just at this resort. Amazing place to relax. Here's the website http://www.thespringscostarica.com/. We did a volcano hike but I'd honestly skip the tour guide and just do one on your own. He told us history of the volcano but it wasn't the greatest hike and wasn't cheap for what we got.
MONTEVERDE: We stayed at Arco Iris! It was great. smaller rooms but very intimate feel and the nice thing is you're walking distance to restaurants, shops, bank, grocery store but once you're in the grounds it feels very lush and private. It's in Santa Elena, which I'm sure you know, and I definitely recommend staying there if you're not getting a vehicle as the other places don't have much in walking distance. We did a guided tour Curicancha which was pretty nice but I would recommend the other 2 parks. If you're really into birds this hike is for you since that's pretty much all we saw (no quetzal on this trip unfortunately). Curicancha didn't have the moist feel of the other ones (like you'd expect in a rainforest) Most of the tour was in a low dry forest that kind of felt like one we'd hike through at home in California... we did the Selvatura hanging bridges tour and it was right next to the Santa Elena park entrance and definitely more of a rainforest feel. I recommend doing a night hike in Monteverde. We did the Silvestra one and we ended up seeing 2 sloths and a green viper, a kinkajou among various insects and sleeping birds. Weather was sunny, warm during the day and cooled down at night. You definitely need some layers here. The wind picks up at night and there was not much humidity.
MANUEL ANTONIO: As for Manuel Antonio, we just got here today. Definitely the beachy tropical vibe here. Weather is warm and humid and has a very laid back feel. We plan to check out the national park & beaches tomorrow. So far we've seen a sloth, an iguana and a couple different breeds of monkeys and we haven't been into the national park yet.
As for the vehicle situation, we rented a 4 wheel drive and have loved having our own ride. Depending on rides and buses can be annoying and having to stick to a schedule etc. We have really liked being able to stop off where we want (for example we stopped and had lunch in Jaco on the way here today)... People freak you out about renting a vehicle here but we've had no problems. The roads into and out of Monteverde are dirt and rocky but if you have a decent 4wd you'll be fine. Just my 2 cents.
I think each place has different things to offer and you should see all 3.
Good luck planning!