You can't get cheaper than free, and so it's hard to be anything less than happy with a visit to…read morethe Botanic Gardens in Planten und Blomen. It's great on a cold winter's day because everything is so nice and tropical in there - the heating costs must be enormous!
However, unless you're massively into plants, then it's not that interesting. My favourite bits were the insect-eating plants, and the cactus room.