Last weekend we went to Inspiration Lake which is part of Disneyland. It's free, open daily, and only a 15 minute walk from the Disney MTR station (although with the actual theme park being right there I guess most people would bypass this outdoor park). I was actually surprised with how big the park was - there is the lake itself where you can rent paddle boats, a kids playground, and lots and lots of green space with actual real grass! that you can picnic in or you can also rent out bikes to ride along the paths with.
The Disney train was super cute, and had Mickey Mouse shaped windows/handles and statues of the Disney characters on display:
Climbing rocks and fun with balloons!