Just add water! If the weather is warm enough I find that a little water play can go a long way to making even the biggest kid happy. A day at the beach with friends or family is great. Or if you don't live near the beach even some fun in the backyard sprinkler, water guns or balloons, or make friends with someone with a pool.
I think that our community spaces, parks, playgrounds, walking tracks are under-rated. Packing a small picnic, taking the kids to the playground, then go for a walk is a nice way to break the routine and breath in some fresh air.
Hiking and biking are great ways to enjoy the day without breaking the bank. Local parks and natural spaces usually have a few great hiking or biking trails but there are usually some hidden gems that can be discovered by contacting local biking societies or hiking groups. These activities are a great way to burn some energy, stay in shape and enjoy the outdoors.