Hi vegfamily, there are no longer any train passes you can get (they had them a long time ago now), but there is the 1-day Brupass ticket that you can buy for public transport within Brussels. It is good for all 4 transport companies (STIB/MIVB, Belgian Rail, De Lijn and LETEC). It costs €9,50. There is also a cheaper Brupass for just the STIB/MIVB company, €9,00 but you have to buy it from a kiosk.
To be honest, as tourists you might use the metro and the odd tram within Brussels, you probably won't use anything but the STIB/MIVB. I would just use my credit card to tap on, on the tram and just pay for point-to-point tickets.
There are also 10-journey tickets you can buy which are quite good value. You can load it onto a Mobib card (an electronic card bought from a machine or a kiosk). That can take you on any public transport in Brussels (Brupass) and around Brussels (Brupass XL). That will last you a few days and might be a better solution if you expect to be using transport a lot.
If you say where you expect to be travelling we can give you a better idea of what ticket/s to get.
Lavandula