Millenium Park is a great day trip destination for families. They have a play area intended just for little ones that is just great. My toddlers had a blast climbing, sliding, and swinging on equipment made their size. The beach area is beautiful. Be sure to bring a picnic!
This park outside of Grand Rapids proper is a fun place to take your children on warm summer days. There are pavilions, grills, and a playground in the "free" area. lf you'd like to enjoy the man-made lake and the really fun splash pad, there is an entrance fee ($4/adult, $2/child, and under 3 are free - or you can buy a season pass). There are also bathrooms with showers that are fairly well cleaned, as well as a concessions area with tables if you'd like to bring a picnic lunch.