Guess that Antwerp belongs to the obvious, right?
There is every hour a direct bus (line 42) from Bruges railway station to Sluis with a stop in Sint Anna ter Muiden, a very charming hamlet. Sluis is very touristy, but cute enough for a visit to my opinion. You wil see there many visitors from Belgium and during the summer period Germany too. There is a windmill often in working order and open to visit, from the platform you can overlook Sluis. This little town has like Damme still it’s ramparts with an encircling bike- / footpath and a footpath on top overviewing the historic centre. Bruges to Sluis takes a half hour according schedule.
What most do is biking from Bruges to Damme, but for a 4 year old not doable. Unless you can rent a bike with a backseat. If so check first it’s safety, if her legs can’t anyway hit the spokes. With strong wind e-bikes are worth to consider, possible for going further along the canal to Sluis. Have to say some stretches are bumpy as are the cobbled streets in Bruges too. In Damme you can climb the bell tower of the church.
Further you can take the train to Lissewege, but as it is a small place a visit doesn’t take long and best to combine it with coastal resort De Haan. For this you have to take the train further to Zeebrugge and from there the coastal tram to De Haan, well known for it’s Belle Epoque architecture. For going back tram to Blankenberge or Oostende with an additional train to Bruges. Will take between a half hour and 1 hour.
As Sint Anna ter Muiden and Lissewege are too small to spend a complete hour, the time needed to catch the next bus or train and is best to combine with a drink in one of the restaurants / cafes there.
If Ypres / Ieper doesn't belong to the obvious it's certainly worth a visit but takes with the train from Bruges one and a half hour.
Getting to other places of interest within the time you prefer isn't doable.
