I went for spring break last year. It was awesome. We went to the volcano in the middle, some sweet hot springs, and a massive zip-line tour through the rain forest which was ridiculous. I don't know anything about the night life or what to do for a bachelor party though cause I was 18 and with my family.
I stayed in Quepos too, the local beaches are sick and there's these chicken joints (kinda like kfc) literally every 5 blocks where you can get all kinds of food for cheap. Make sure you go whitewater rafting and ziplining, you'll see monkeys and sloths everywhere haha.