How long of a trip are you looking to take?

Including overnight trains, your possibilities are endless. But if you're looking for a day trip, your options are somewhat limited thanks to Amsterdam's less-than-central position on the continent and the lack of high-speed rail in the Netherlands.

There are a couple cities outside of the NL that fit your criteria -- Antwerp is definitely one. It's a walkable city with great pubs, a festival-happy center and beautiful scenery. 2 hrs / I think around 40 Euros round-trip by train. I'd recommend it over the other ex-NL choices (Brussels, Bruges, Dusseldorf).