Something off-the-beaten-track in Bratislava, which I wouldn't call nice, but definitely worth to see: Bratislava has a cute historical city center, but it is also one of the central/eastern European capitals most badly hit by the communist "urbanism". Petrzalka - the part of Bratislava south of the river Danube - is an excellent example. You can see it very well from the castle, but if you have some time, take some bus that will take you thoroughly through this part (e.g. 68). If you don't like crowds packed in a very limited place, then do it in off-peak hours (summer time used to be less critical). Otherwise don't worry - it is absolutely safe :-)
On the other side of the Danube from Old Town is the suburb of Petrzalka, home to 150,000 (1/3 of the population of Bratislava). You can see the big concrete towers in the picture behind Novy Most.
Petrzalka was built after WWII by the Communists to house the workers they shipped to Bratislava. It was surrounded by barbed wire fences and lookout towers where armed soldiers would shoot anyone trying to escape over the border to Austria. Over 300 people were killed this way. It also had a very high suicide and drug abuse rate under Communism.
It`s not really a must to visit but it`s part of the Communistic past: Bratislava`s suburb of Petrzalka! In every town of all former RGW countries these large apartment blocks were built. Some of them are up to 300 meters long. About 150.000 inhabitants are living there.
A good website is: http://www.petrzalka.sk/
(only in Slovak)