Riverside and outdoors
Boulters Lock and the Thames Path give you flat pushchair friendly walking with boats to watch and ice cream at the end. Ray Mill Island next to the lock has gardens birds and a playground and costs nothing.
Braywick Park and Kidwells Park both have good play equipment and Kidwells sits close enough to the town centre to combine with shopping.
Rainy day options
Braywick Leisure Centre has a teaching pool and family swim sessions. Soft play and trampoline parks on the edge of town cover the wettest afternoons and Maidenhead Library runs regular free story and craft sessions for younger children.
Easy days out nearby
Windsor Great Park Legoland Cliveden and Odds Farm Park are all within a short drive. Cliveden in particular is worth a National Trust membership if you are staying in the area.
In short
The river the parks and the leisure centre cover most weekends with Windsor and Cliveden for something bigger.