Schwimmbad Musik Club is the most well-known disco in all of Heidelberg. It boasts two discos, one live stage, and a movie room. It is a rather laid-back club where you can go with a group of friends and feel very comfortable. There is no need for dressed-up clubbing attire (i.e. black pants or "Sex in the City" attire). Be sure to check their website at http://www.schwimmbad-musik-club.de to see what the night's music type and band will be. Cover charges can vary, but are usually under 5€, unless you're trying to see the live band. Drinks range from about 2€ for beer to 7€ for fancy stuff. Parking is usually not a problem, as they have a large car lot. The party picks up around midnight and lasts into the wee hours of the morning.
Many Americans living in Heidelberg enjoy hanging out at Schwimmbad because of its laid-back atmosphere. If you're between the ages of 16 and 18, you will be allowed in until about 1am, but you must leave your ID at the entrance and pick it up as you leave. Over 18, you're in for as long as you like.
One piece of advice: Wednesday night is "Goth Night" in the downstairs club, which may intimidate some party-seekers, but if you can get past all the pleather, there is lots of fun to be had (you might just want to gear your wardrobe towards black). The upstairs plays alternative that night, so any type of grunge is cool. All other nights are more traditional "club" nights.