Lavery's bar is great for atmosphere with three diffrent bars, pool, music etc. The Crown is pretty and well worth a visit but you wouldnt necessarily want to spend a whole night there. The Felon's bar on Andersontown road used to be a republican drinking club (still is upstairs). Open bus top tour of troubles sites is good. Out of town Portrush and Newcastle are worth a visit if you have a car - and tour the Bushmills Distillery. Also miles of empty unspoilt beaches north of Belfast. Also visit my uncles bike shop, McConvey's on Ormeau Road if you want to rent a bike:)
http://www.laverysbelfast.com/
http://www.rentabikebelfast.com/index.php
http://www.bushmills.com/Visit