I'm arriving via River cruise in Brussels on Sunday morning, March 31st. Have to be in Budapest on Tuesday, April 2nd by 2pm. Trying to decide what to see in between these 2 cities and my best choice of travel, ie air or train. I will already have time in both these cities so want to see another city in between. Thanks

Prague is the logical city. I'd go wit a low cost carrier like Ryanair, Easyjet or ...

Vienna would also be convenient but is a long train ride from Brussels but there is an overnight train from nearby Germany I believe - could be a nice experience if can sleep well with modicum of noise. Check www.bahn.de/en - German Railways site for schedules and fares, including special discounts if book early enough to get them. For lots on trains and booking own tickets check www.seat61.com; www.budgeteuropetravel.com and www.ricksteves.com. Munich of course would also be a natural stop.