If you're looking for some family fun in northern Alsace, you'll find quite a few day trips from Strasbourg here.
Northern Alsace family fun activities are all a short distance from Strasbourg.
I'll add more things as I discover them...
Or you can just read everyone else's recommendations...
Haut Koenigsbourg is a castle originally built between the eleventh century and the sixteenth century that was later destroyed and then restored (with a lot of historical research) in the early 1900s. It's in the mountains and our kids loved hiking around the outside of it as well as seeing a REAL castle (prince and princesses not included).
The Parc des Cigognes is sort of a tiny little Seaworld for the regional animals in Alsace. It's more than a zoo with animal shows. They also are active in protecting some of Alsace's endangered species. You can have a little fun and feel good about donating to a worthy cause at the same time.
If you like monkeys, the Montagne des Singes is better than any zoo! It's actually a research and conservation center for Barbary Macaques (an endangered species). Most importantly, you get to pet the monkeys! Because these monkeys have been living here since 1969, they are not afraid of humans, so you get a real up-close-and-personal experience.
The Volerie des Aigles is a great way to get up-close-and-personal with eagles and other large birds of prey. They have been breeding and protecting them here since 1968. They put on a great show right next to the castle of Kinztheim, so you can kill two birds with one stone (excuse the pun ;)
If you're going to be here during the Christmas holidays, you might find outdoor ice skating rinks in the town or village you are staying in. They can get crowded (so try to go as early as possible), but the kids don't care. They love it!
If you have the time, you can treat the kids to something they'll really LOVE! Europa Park is a mix between Disneyland and Six Flags and it's only an hour drive or less from anywhere in Alsace. It's in Rust, Germany. There's plenty for everyone no matter what age they are here. We all love it!
Strasbourg has a Zoo!. Well, it kind of has a zoo. It's more like a petting zoo. If you want to visit a real zoological park, you'll have to go to Mulhouse or Basel in Southern Alsace. But it's only a short train ride or car ride away!
Almost every village and most towns have walking, cycling, and skating paths. When the weather's nice, we often go for a walk or even better go skating! If you are bringing your bikes, all the better, but you can probably find somewhere to rent whatever you need if the area attracts tourists on a regular basis. Your hotel should know where to find them.
Our kids find Bowling interesting. Be careful though. Not all bowling alleys in Europe are kid-friendly. A lot of them are like pool halls but with bowling lanes too. Check with your hotel. They may be able to point you in the right direction.
You can find Go Kart Racing Tracks in Alsace. Some of them have other things like Bowling and Mini Golf too. There are also stand alone Mini Golf courses to be found, but it's just normal mini golf, not a Pirate Adventure Mini Golf or something like that.