thermal bath
Go for a thermal bath. The city is named Baden to bath/bathe because of the natural geothermal springs were originally ducted into baths by the Romans. The springs are claimed to be the most mineral rich in Europe. The main baths are in Thermalbaden and cost 16CHF + 5CHF returnable deposit , for an adult. Lots of luxury hotels are located around the thermal area by the river. Many hotels have private thermal baths. And you should go!
Children's museum
There is Children's museum, KinderMuseum (http://www.kindermuseum.ch). Could be an interesting activity for children.
Most Saturday Mornings there is a local food market Farmers Market in Weitergasse and sometimes special events e.g.cultural or sports activities or special markets go on in front of the Train Station Bahnhofplatz- Badstrasse.
swim
During the summer, walk along the beautiful Limmat river. It is recommended to go for a swim on sunny days when the current is not too strong, however, be wary of the nearby dams. There should be warning signs if it is too close to a dam, but to be sure, you'll know it's a safe area if there are many locals nearby swimming as well.