I haven't been to the gardens of Belgium but here are some tips on researching gardens on the web.
I usually start with
Garden Visit website. I noticed that it has been updated and is using more modern webtools like google maps. It will include links to the official websites of the of the gardens.
Next, I google the gardens directly. This will often take me to organized garden tours or other gardening websites. These will often have information on what to see and also mention other gardens in the area. The tour itineraries might also include private gardens but I still find the organized tours good for planning my own trip. I've used this technique to organize wildflower/garden trips to both South Africa and Western Australia.
Here are a few other websites that I found that might be of interest for your planning:
Visit Belgium Garden page Belgium Open Garden website - potentially there will be an open garden while you are visiting.
Jardin d'Annevoie - This seems to be the best garden in Belgium. It was listed on most of the tours.
We did visit one unusual garden/botanical place in Belgium.
King Leopold Royal Greenhouses are open to the public during the spring. The days are somewhat similar to the period that Keukenhof gardens are open. It was rather crowded the day we visited but they were interesting. You are not free to roam in the greenhouses but follow a route through the different houses. The final room is the large Palm house which was pretty nice. Not as nice as the glasshouses at Kew but I'd rate it worth a trip especially if you are in the area.
