Let's go out to the Park of Saint-Germain-en-Laye!
The park is easily reachable by RER and there are good parking places around. We could bring a kite if there will be wind, and a badminton set if there will be no wind. We might bring our bikes if some would like to ride them. We could bring some card games as well. And there are several other options. If some would like to come earlier or stay longer it would be still great.