This (the Veletrzni Palac) is easily the most under-rated and from the (many) times I've been there over the last 20 years, the most under-visited major art museum in Europe, probably because of its location outside Prague Old Town. The collection, and semi-permant display, following a re-hang, is modern and predominantly (as you would expect) Czech. This three-floor exhibition has been re-arranged superbly and, unless you have been before, you should expect art from the last 150 years that you have not seen previously. To really see what is in this building you need around six hours - I did it in three wet afternoons at roughly two hours per floor. Or you could do a whole day including coffees, lunch and a well-deserved pre-dinner drink in the modern and super popular cafe. Go! A tram stops close by or the Vltava metro stop is only a few minutes walk. A tip on the cost, you can buy a combined year long friends ticket which gives unlimited access and is excellent value, particularly if you want to do this site and others over a few days. Remember too, the mediaeval branch in Josefov, the fine arts one near the Castle, the temporary exhibitionat the Kinsky on Old Town Square and so on, all of which are worth a look.