The center of Mannheim is laid out like a chess board, with no real street names. Addresses in the Quadrat take the form of a grid reference, such as Q3, 12 designating a block. Note that the streets themselves are not named, rather "Q3" refers to the block itself. If you follow a street from Q3, you might end up at either Q2 or P3. It is best to navigate by "following" the blocks rather than the streets. If you get lost, a rather high probability, simply ask a local. They are used to it.
Mannheim was a small fishing village before it became a city at the beginning of the 17th century. It was constructed on the site of a fortress guarding the confluence of the rivers Rhine and Neckar. Even now a few remnants of the fortification can be seen, and the peculiar street layout owes to that part of its history. For 58 years, Mannheim served as a royal residence and gave Schiller, Lessing, Goethe and Mozart a home for some time. Before World War II Mannheim was a beautiful city, but was flattened in bomb raids due to its industrial significance. When it was time to rebuild the city, Mannheim, like many other German cities, opted for an all out modern approach to urban development. Thus, most of the old quarters were replaced by buildings typical of the 1950s. If you are not an adept to architecture, their appeal might not be easy to grasp. As a result, the impression is more of an industrial city with a few spots of beauty.
Modern Mannheim is the second biggest city in Baden-Württemberg and one of the hotspots of immigration. Because of that you'll encounter a lively and colorful mixture of nationalities and cultures in the city. The Mannheim/Heidelberg area hosts the largest concentration of US military personnel in Germany, and barracks are found in many of the suburbs.
Get in and away
Transport from Frankfurt airport, to Mannheim is by ICE high speed train (30 minutes, €25), or Lufthansa Airport Shuttle bus (60 minutes, €35). The Lufthansa Airport Shuttle may be ticketed together with the airfare and Lufthansa (also Condor, SAS or South African Airways) passengers can have luggage checked in directly to the final destination. The shuttle arrives/departs at the front of Dorint Kongresshotel on Friedrichsring 6 near the Water Tower (Wasserturm). The old departure near the central station no longer exists.
Mannheim is a regional transport hub with ICE, IC and regional trains all stopping in Mannheim Hauptbahnhof.
Mannheim is served by Eurolines (Deutsche Touring) with overnight long distance services to destinations in France, UK and other neighboring countries. The bus station (ZOB) is at Heinrich-von-Stephan-Str, near the Main Station (Hauptbahnhof).
The public transportation system is quite extensive. Bus routes cover Mannheim, and the street car system connects Mannheim to Ludwigshafen across the river, Heidelberg a few minutes away, and Weinheim, in addition to major routes across and through the city.
The National Theater  has a different show almost every night (for some shows, like ballets or opera, the language barrier is not an issue). The street car stops right outside the theater, and student tickets are much reduced (5 or 15 euro).
The most parts of Mannheim are safe, but there are a few of districts that have higher crime rates. Examples are Vogelstang, Neckarstadt-West, Jungbusch (night) and some others. Street crime and violence, however, are very rare, so you will be perfectly ok if you simply use your common sense. In particular, it is not dangerous at all to visit the pubs and clubs of the Jungbusch or the Neckarstadt.
Blau (german for "blue") is the favourite hangout for leftists, post-punks and alternative culture adepts. It is also here where you are likely to run into activities of the "Büro für angewandten Realismus" (office for applied realism), a group of artists that organise cultural events every now and then. Additionally, there are displays of their artwork in the pub. Jungbuschstrasse 14, 68059 Mannheim
The Onyx is bustling with activity almost every night after normal working hours. They offer a full bar and excellent menu for breakfast, lunch, or dinner. It is located near the Wasserturm (water tower). Typically more dressy than other hangouts. Friedrichsplatz 12, 68165 Mannheim. Tel: 0621/1286888
Café Bernstein is a nice french style Bar/Café that offers a good selection of beers and wines. They also offer a small but fine selection of lunch/dinner. Reasonable prices. Exceptional friendly staff! Bernstein is located in the Schwetzinger Vorstadt. 10 min walk from Main station. Seckenheimer Straße 58, 68167 Mannheim. Tel: 0621/4949159
Café Odeon is a nice Bar that belongs to an alternative Cinema. Reasonable prices and relaxed people.G7,10
SOHO Club is a small club with reasonable prices, good music and relaxed guests between 20 and 40. Music varies from night to night, see Homepage for details (http://www.soho-club.de/). SOHO Club is located on the Ring-road that begirds the inner city. Don't miss the cocktail happy hour until 11 p.m. J7,16, 68159 Mannheim. Tel: 0621/13382
Murphy's Law Website/Calendar of Eventsis a great Irish pub that serves up Boddington's and Kilkenny on tap (a rare find), in addition to the usual suspects. The pub fare is better than most, especially the Irish breakfast, chili, and fish and chips. It's usually packed on the Weekend nights with English, Irish, Welsh, Scottish and American ex pats and a few Germans (typically University students) thrown in there for local flavor. Just a hop and a skip from the main train staion. Weeknds, Fall to Spring usually feature live music. Tuesday is trivia night. Be sure to say hello to John at the end of the bar. Kaiserring 10-12 (Bahnhofvorplatz), 68161 Mannheim. Tel: 0621/1563925
You might also want to have a look at http://www.schneckenhof.de (German language), which has a detailed Nightlife guide. http://www.meier-online.de has a calendar and guide for all kinds of events and locations in Mannheim, Heidelberg and Ludwigshafen.