My friend and I went to the Riu Palace for 6 days at the tail end of March '15.
Weather: We did a lot of research beforehand, and chose this time because it's the "dry season" which means no rain. And it was accurate, because it didn't rain in Guanacaste once. It was beautiful (95 degrees) every day, and mostly cloudless save for maybe one day.
Kids: We also stayed at the Riu Palace instead of the Riu Guanacaste because of the kids. We were told the Riu Guanacaste has a TON of kids, and when we walked over to their pool one day we confirmed it: KIDS GALORE. We're not kid-haters, but we don't want to see kids on our vacation. Period. That being said, there were families at The Palace, but nothing insane. Maybe 25 kids total out of hundreds of people. Also, kids weren't allowed in the pool with the swim-up bar (a rule that was enforced) so we spent most of our time there.
Food: The food was very good. We ate at all 4 restaurants, and the buffet. The Japanese restaurant was great, the steakhouse was good, and the Fusion restaurant was great. If you are Italian, then stay away from the Italian restaurant. Amateurs. The buffet was good - my boyfriend was able to have the chef make him his own pasta and he loved it. I wasn't as impressed with the buffet, but it was definitely good. It's the same restaurant as the breakfast buffet, and we always got an awesome omelette and the usual (eggs, bacon, hashbrowns, pancakes, french toast, juice, coffee, tea, etc). Every day they had lunch at the pool which was AMAZING (big juicy bbq chicken legs/thighs, tortillas, steak strips, chicken, pork, rice & beans, guac and hot sauce, etc). They always had awesome speciality drinks (one day it was out of a coconut! This was from 12PM-1PM and always my favorite meal of the day by far.
*Make your reservations the second you get to the hotel. They are open 9AM-2PM I believe, and you can make reservations up to 2 days in advance. One day we walked into a restaurant without a reservation and they said they had a cancellation and allowed us to be seated, but that was dumb luck. The times to eat are 6:30PM and 8:30PM I believe, but if you're not the scheduling type, stick with the buffet.
Beach: The beach was VERY disappointing. Since the resort is near volcanoes, the sand is dark and dirty-looking, and the shore has a LOT of rocks as you're walking into the ocean. And speaking of the ocean - UGH. DO NOT picture the Bahamas. At all. Not even for a second. If you're from the East Coast, picture the Jersey Shore. I wore goggles one day and never took them out again, because I couldn't see my hand in front of my face under the water. Total bust. We still went every day, because it's still the beach and the water was warm/cool so it was still refreshing, but we ended up spending most of our time at the pool, which I'll write about next.
Pools: There were 4 pools (5 if you count the infinity pool as two) since it drops down into another pool. The two on either end, as well as the middle infinity pool (which was beautiful for pictures) were inhabited by families. Again, not too many kids, but many of the older couples and families stayed there. My boyfriend and I mostly stayed in the swim-up bar pool, because the bar was never crowded (got drinks easily with no wait). Make sure you ask for your drinks to be strong, because ours barely had alchohol in them. (I like them that way but if you don't, speak up.) We loved the underwater cement beds that the Riu always has. Great for tanning and staying cool at the same time - plus there are little cement tables for your drinks and sunglasses. We always put our towels down as a little pillow early in the morning to save our spots. Those beds were in all the pools and there were always free ones.
Excursions: We booked through TAM (our booker was Melvin, who's freaking awesome). We were torn between a few things and ended up doing the Arenal Volcano and Hot Springs Combo from 7:30AM-10PM. We did this because we were told that you absolutely cannot go to Costa Rica without seeing the rainforest. We drove on a bus (which was not too comfortable so bring your neck pillow) 4 hours to the volcano, but it was cloudy so we couldn't see the top womp womp. We did, however, see coatis (in the road) and monkeys (in a tree on the side of the road), which was sooo cool. Then we went ziplining. I hated it because you have to break yourself with you hand, and I was petrified my finger would get chopped off (you're going really fast) or my arm was snap off (when you break your arm rips back). But our group and my boyfriend liked it a lot. Then we got to go to a 5-star resort in the middle of the rainforest called The Springs Resort. It was literally the most amazing thing I've EVER seen. It was built IN The rainforest, so as you're sitting in natural hot springs (ranging from very warm to jacuzzi heat) you are looking at awesome flowers, trees, plants, birds, etc. We took about 100 pictures. Next time I go back to Costa Rica I am DEFINITELY staying at a rainforest resort minimum 3 days before driving over to the beach. After the hot springs, we had a free dinner (delicious) overlooking the rainforest and the volcano. I was in photo heaven.
*I know people who have done the riverboat cruise and the National Park and loved it, because you get to see monkeys, alligators, etc. But we just didn't have enough time. I also heard the Marina del Rey catamaran is awesome and there's a chance of seeing whales.
Scubadiving/snorkeling: Don't do it. As I said earlier, the beach is NOT nice. So there's no point in wasting money to look into water that is not clear. We met a couple who went snorkeling and saw one brown fish, so we decided "Hey, if snorkeling sucks, scubadiving must be better." Wrong. Cost $99 instead of $25 and it SUCKED. It was my first time and I didn't think the guide was too nice (I needed a lot of help and he had that "Ugh, why isn't she GETTING this?" attitude). It was my boyfriend's second time (his first was in Cozumel, Mexico) and he said this was a joke compared to that. We saw exactly two fish, and when I asked the guide if that was normal, he said "We took too long on the surface," which I guess was a dig at me and the other struggling first-timers of the group. But I highly doubt it, reading about it from other people who've tried it in Costa Rica. Stay away.
All in all, we had an amazing trip. We wished we had done 9 days instead of 6, because we would have liked to do one more excursion and also had a couple of extra R&R days at the pool/beach. I also wished I brushed up on my Spanish via Rosetta's Stone, because the workers spoke VERY little English, which didn't matter too much, but it still would have been nice to communicate them a little bit more. Also, we had free wifi EVERYWHERE which was great because I love Skyping my family back home when I travel and Instagramming my photos for all my friends to see and get appropriately jealous!