We were a little older than you on our first trip (late 20s). We also stayed at RIU.

Pack a back pack. Get a cab to Margarittaville. Walk through to the beach. Hang a left and have good time. Get there around noon. Walk the beach. Stop wherever you want to drink. If you feel adventurous, walk all the way down the beach, through the park and into town. Stay to the right and head up the west end road for more bars. Do this the first day or 2.