Eastern Belgium is famous for the battle of Bastogne, beautiful forests, and the original Spa! Also be sure to check out historic Liege, home of Fort Loncin (which is open for something like 8 hours a week). It’s also famous for the delicious Liege waffle.
Spa: Thermes de Spa is known as the world’s first spa — it’s where the name comes from. You can take a funicular up to the spa from the old city, and can either buy a day pass to just soak in the waters / sauna, or book a wide range of treatments– including a peat bath, as well as a silly one where someone stands there and sprays water at you. The treatments are super popular though (including the water-spraying for some reason), so its best to book early. Naturally, Belgium’s famous Spa water comes from here. Given that it’s a resort town, there are many fancy / organic restaurants around the area.
Leuven: A lively university town with its own mini Grand Place, this is a great stop-over while in Brussels. It also hosts the Zythos beer fest, as well as the Amstel factory.
Tongeren: A small medieval town in eastern Belgium, it features a huge antiques market on Sunday mornings. Worth a visit!