I am personally not a huge fan of all inclusives. The big chains are usually pretty generic, so you

It's our first trip there and we are looking for snorkeling, night life (but safe) for a bunch of girls, sight seeing, water activity, outdoor fun- hiking and any must see things we need to do.. Should we do AI?? What's a good location for hotel and do we need car rental??
I am personally not a huge fan of all inclusives. The big chains are usually pretty generic, so you don't have a local feel. Often, western caribean cruises the only locals you meet are the waiters and bartenders. Plus, most of the all inclusives are kind of tucked up into the northwestern part of the country. None of these are very close to the natural areas you typically see on TV shows covering Costa Rica.
I would look at either Manuel Antonio or Mal Pais / Santa Teresa. FYI. Neither of these are great snorkel spots. Lots of rivers and estuaries drain into the Pacific here, and cuts the visibility. Also the currents are so active, that the coral doesn't grow like it does in the Caribbean. There is snorkeling offered. But I have never been overwhelmed by it.
Manuel Antonio would be the easiest to get to from the SJO airport. Plus, the area is pretty compact. Once you are there, you will be able to get around with taxis pretty easily. There isn't much club style nightlife here. But there are some fun restaurants and fun smaller bars. Lots of nature / adventure tours in this area too. So you can go to the beach, or whitewater raft, or take a zipline tour during the day. Then get a great meal and find a small open air bar to hang out at.
Mal Pais / Santa Teresa has a really fun and funky surfer vibe to it, and a young tourist population. We have been here several times over the last few years. We have always noticed lots of younger folks strolling on the street. Yoga is a big thing here, and there are lots of surfers. Lots of people western caribean cruises in great shape.
The water is turbulent here. So if you are not a surfer, you have to pick your spot to swim. Great restaurants in the area as well. Most of the ex-pat population is Europeans or South Americans. So the food is always interesting.
The one issue with this area, is accessibility. You have to take a ferry from Puntarenas to get here. So it is a little more time consuming, and a little more expensive. But if you are spreading the transportation cost between several people, it won't be a huge deal. The Ferry ride is fun too.
What you suggest for a hotel/location spot? We are looking for some active life but also some relaxation. We want to make the most of our time and see the country life. You read and see so many pictures but now planning the best trip is hard as we are unsure of where to stay and do.
Check out Tulemar Bungalows in Manuel Antonio. We loved it there; they have their own beach with boogie boards, paddle boards, kayaks, etc. Great views. Tons of wildlife. You can do a tour of the national park, do a mangrove tour, see monkeys, crocs, sloths, etc., do a spice tour, zipline, western caribean cruises raft, and visit some great bars and restaurants at night.
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