It's best to get a map because it's easy to get lost if you stray too far. The L train takes you to Bedford (1st stop in brooklyn). This is ground zero. Go for the driggs exit and turn left. The Abbey has happy hour 4-8 and a pool table in the back. Back to Bedford, one block west of Driggs, turn left, on your left is Vera Cruz. 5-7 happy hour. Cheap Mexican Food (it's good and I'm a native californian) and discount Margaritas. Pete's Candy Store on Lorimer is a bit of a hike (check your map) but they have a good happy hour too. The blue lounge on Driggs and N9th or N10th (you can't miss it, it's got a big blue sign) has a happy hour till 9. They only serve beer, wine, and sake. The back room is really mellow. If you get hungry late, go to Yabbi's, excellent food and vegitarian options. On bedford and N1st, it is a converted gas station.
Also check out Black Betty's on Metropolitan and Havemayer. Mostly a pick up bar, they frequently have very good live music.
If you feel like you have deep pockets, go to Peter Luger's Steak House on Broadway. Probably the best steak in Greater New York. Make reservations far in advance. Cash only.
If you've got a big group and are looking for a nice place to start out, go to PlanEat Thai. Take the Beford stairs out of the subway and walk straight. It's a half a block on your right. Incredable interior decor. Reasonably price Sushi and Thai food. The service is a little off because there's no co-ordination between the sushi bar and the kitchen. Still, the food is very good.