On our trip, we arrived in Brussels late one evening via train, spent the morning in Brussels, spent the afternoon in Bruges, and the evening back in Brussels. Overall, we probably spent a total of approximately eight hours touring Brussels. During our time there, we pretty much stuck to walking around "the core" (Manneken Pis, Grand Place, Galeries St. Hubert, Rue des bouchers) with the furthest we went being the Fish Market area. We didn't devote a lot of time to sit down meals other than maybe an hour at Chez Leon for lunch. For what we wanted to see in our first trip to Brussels, we found this to be enough time and met our goals. Ideally, if we had more time, I would have like to visited other parts of Brussels but we wanted to allocate more time to other cities during this trip.

The Manneken Pis is a very quick walk from Grand Place. I can't see one spending more than 15 mins there though. Recommend bringing a mini tripod for taking pics of Grand Place at night. If you can get in earlier, I'd probably recommend it just because places have different vibes during the day versus night and it's nice experiencing both.