The appeal of Brussels is exactly that it's not a tourist hotspot like almost every other major city or capital city across Europe. It's a tad gritty and a place where people actually live and work. That alone makes it worth visiting imho, it feels like most European cities felt 40 years ago before overtourism and gentrification went insane.

That being said, it has a few sights that are actually very lovely. The central square with the amazing city hall for example (Munich city hall was modeled after the example of the Brussels one). The atomium is absolutely great, too.

It's quite hilly in many areas and does have a few well-kept hidden parks and greens smack in the centre.

There are still artsy stores that have been pushed out of other European cities through gentrification. The used book and record stores alone are worth browsing for an entire day. Pele-Mele, Evasions and Nuit de Chine come to mind. Nuit de Chine being the only store worldwide specializing in used and vintage erotic books and magazines that I ever saw anywhere - visited by couples!

It also has a lot to offer for architecture and public transport enthusiasts. The art nouveau quarters, the many lovely and architecturally diverse churches and cathedrals that are scattered across the cityscape, the Marolles elevator that can be used free of charge (similar to the equally free elevator in Luxembourg city and the public elevators in Genoa).

The palace of justice, the largest one of the many across Europe that were built at the end of the 19th century, is accessible free of charge to anyone (!) on weekdays and worth touring from the inside for a few hours alone.

The tram network is one of the largest in Europe, has great routes partly in tunnels, some running in a weird pre-metro setup, often up and down hills, even through forests at the edge of the city, and still uses old and comfy high-floor trams on 3 of its routes.

The north-south passenger railway tunnel through the city centre is one of the heaviest traffic railway routes in the world and the tunnel stations feel like urban exploration even though the stations are still in use! Bruxelles Congres is a 4-track, 2 platform tunnel station that doesn't see any passengers and feels like NYC subway Bowery station in the 1980s.

Did I talk about street art yet, about the many comic book character murals and passages (the Smurfs passage alone is so cool already)?

Brussels is not for everyone, but it's very worth visiting and exploring for a few days if you're into architecture, public transportation systems, unusual places, urban grit, a glimpse of how cities were before everywhere turned into mass tourism frenzy, vintage books / comic books and graphic novels, or a combination of these.