I'm from Belgium - just putting that out there so you can put things into context.
If you want to go walking: do not go to Belgium, Northern France, the Netherlands. I mean, it is a nice area, and it is peaceful, but it is not astonishing. If I were to go somewhere for walking and enjoy nature, it would most probably be Ireland or Scotland, possibly Wales or Iceland. German forests are thick and very atmospheric as well. In the South I can advise you Ile de Ré
, which is very nice. I have spent two weeks in Jersey (not New Jersey, mind you) off the coast of France and it is very nice as well.
If you want culture/history, any country will do. Every country, every city even, has its own cultural habits and customs. Personally I think our own, Belgian, culture is pretty nice - knowing that we only got our independence in 1830 and that we have been ruled by Prusians, French, Dutchmen, Spaniards and what not over the centuries.
If you want to go to festivals: Belgium for Rock Werchter, Pukkelpop, Groezrock, Blues Festival Peer. In the Netherlands they have Tommorowland (electronic music). Probably best-known is probably Sziget, which is a long festival in the summer in Budapest.
If you want to chill
and just lie at a beach. Spain, southern France would do.