I was at the Riu Palace last month in Costa Rica along with my girlfriend and a large group of people. We had a fantastic time and truly enjoyed this resort hotel. The employees are fantastic and will go out of their way to keep you happy. I agree with many of the postive reviews, but will offer a few tips from my own experience.
When we were there, you still had to make reservations a day or two in advance for all of the restaurants except the Italian restaurant. I went to all of them with the exception of Krystal, the gourmet fusion restaurant, as it seemed to be a tough reservation to get (and my girlfriend wasn't impressed by the eclectic menu). The filet at the steakhouse was fine, though the halved lobster was a bit overcooked. The Japanese restaurant had very tasty fare, but small portions and not a lot of true raw fish. For raw fish and endless portions, I'd look for the ceviche at the Arenal buffet, or the sushi they occasionally offer there for dinner and at the steakhouse for lunch. The Italian restaurant features a nice appetizer buffet and very good main courses. Prior to choosing any of the specialty restaurants, I'd check the board outside the buffet in the morning to see the theme for the night, as it may be more to your liking. The variety there is always good, so long as you take notice of the special offerings each night. The breakfast at the Arenal was always a treat as well, whether eating light or making a big meal of it. The room service menu is limited, but we never tried it having all the other options available. The 24 hour self-serve in the lobby was nice for draft beer, early morning juice and/or fruit, but much of the food provided seemed to be buffet leftovers.
The gym is very nice for a workout for those who wish to stick to a fitness routine. You can also reserve time in their hot tub at no cost. During the day, there are pool aerobics and other group fitness activities. If you like to swim laps, the pools on either of the ends are long enough to do so and so long as you get there between eight or nine in the morning (till around ten am), you'll have plenty of room before it gets crowded. No buses filled with locals on the weekend, as some complained about earlier. Perhaps they confused the Palace with the lower priced Riu next door. We did go over there to check it out, and it was a lot noisier with more kids and a younger crowd. I would say the only reason to go over there from the Palace might be to catch a game at the sports bar (which the Palace doesn't have). Oh, and for sports fans, there is no ESPN of any kind on the room televisions.
Now as for those complaining about a long walk to the bathrooms from the pools... it amazes me. The steakhouse bathroom is maybe a hundred feet or less from one of the pools, and not far from the other pools. Though signs say not to wear wet clothes or bathing suits in the restaurant, they mean for lunch or dinner. The bathrooms are on the far end of the restaurant by the pool just outside and separated from the seating/serving area and you can certainly use them while in just a bathing suit coming right out of the pools. I would imagine the complaints in this regard are from those who either did not know they could use the steakhouse bathrooms or, judging by the girth of the average vacationer on site, are put out by walking an extra hundred feet.
We used the Gecko tours on the beach for a private tour catered to what we wanted to do. Our guides were most helpful and patient taking us to the volcano park where we swam in the waterfalls, had a hot spring and mud bath and visited other tourist attractions. Don't be afraid to negotiate. Now if nature is your thing, right on site of the resort however, you can see monkeys in the trees in the early morning or late afternoon (near the "Monkey Bar" off the trail from the beach), and if you're lucky (as we were) you may catch a sea turtle digging a hole on the beach to lay eggs. You don't have to go zip-lining through trees, taking a boat through the further off rain forests or heading up north to Nicaragua. There's plenty to do on site and near the hotel to stay active and see the sites.
If you are a U.S. cigar smoker and crave real Cuban cigars, bite the bullet and buy them at the duty free shop when you get off the plane at the airport prior to arriving at the hotel. The Cohibas sold on the beach are all 100% counterfeit no matter what the box looks like, with some containing what can best be described as moldy banana leaves. Many sold at Coco Beach and other tourist shops are counterfeit as well. If you don't know what to look for in a legitimate Cuban cigar, you're probably not going to find one anywhere else but the airport. And while we're on the airport in Liberia, though souvenirs aren't overpriced, all their food is. Seriously, you will pay thirty bucks for a small wrap, a soda and a cup of coffee.
As for tipping, it's not required. This was my first all-inclusive and I was reluctant to tip at first. However, I brought a wad of single dollar bills and left a buck on the nightstand for housekeeping each morning (and sometimes in the fridge for the mini-bar stocker) and tipped bartenders and favored meal servers a buck or two when it felt right. They were most appreciative and I only spent about thirty bucks doing this for the week. A small price to pay and the bartenders will get to know you and go out of their way a bit more to get to know what you like and have it ready for you without even saying a word. They'll even slide over one of their favorite concoctions at Drunk Pool (which is how we affectionately referred to the swim up bar). Of course you don't have to tip... and again, I was reluctant out of principle for the "all-inclusive" price... but I'm glad I did; even on such a small scale.
I hope this is helpful and that those who vacation at the Riu Palace enjoy themselves as much as we did.