Every year during Spring time, the Royal Greenhouses of Laeken open their doors to the public for 3 weeks. This majestic green oasis right outside the heart of Brussels is part of the royal palace and is definitely a must visit when you're in the area during that period.
There's 2 walking tours: a 1-hour walk that takes you straight to the greenhouses and a 2-hour walk where you also stroll through the exterior gardens. While the greenhouses definitely are the most impressive part of your visit, I absolutely do suggest to also take in the outside views if you can.
Of course you can spend as much time as you'd like admiring the exotic flowers and plants, as well as the remarkable architecture of Alphonse Balat, mentor of Victor Horta. The standard entry fee is €5, which I believe this unique experience is well worth it.
