A beautiful little fishing/surfers town. Nice place on the beach, coming up tourism, but still really pure. If you are here stay in arena caliente, it's in the same street as you get off the buss. Or find a nice cabinas. Arena Caliente is a great place, especially when you like surfing, a lot of local surfers are chilling here, so that gives a nice atmosphere. You get your own room for more or less 6 dollars a night. There's a kitchen and you share the toilet and shower. It isn't a new building, not for tourist with money or who likes luxury! So if that's where you're not looking for it's a great place to stay!
In the night you have to go to the iguana bar or tsunami's! If they are closing you've still the sportsbar thats open, it's in the same street as the market(a great market with great comedores with really good food for a nice price!) You've also happy hours in la pelican, a really nice luxury hotel with a nice view, at wednesday and friday nights(maybe it's changed so ask the locals)!
If you want to learn spanish as well, you can arrange this over there, i will recommend to stay at a host family so you will get to know the life of a nica family! I can't remember the name of the school but you have a really good school right on the beach for a good price!
San Juan is a great beach place, to get in touch with the locals, learn spanish, to surf or learn to surf, and to enjoy the rum of nicaragua!