About 45 minutes from Tokyo's skyscrapers, resting in the hinterlands of Tachikawa there is a land of mist and dragons. It's a place where rolling hills tumble toward an Aztec pyramid, and children bounce on clouds. The place is called Showa Kinen Koen, and an afternoon there is time well spent.
Take the train to Nishi Tachikawa station and walk in the main entrance. The park is massive: over three times the size of Yoyogi Koen, which covers 54 hectares in downtown Tokyo. Rental bikes are a great way to get around the place, and you'll find rental stations near all three main entrances. Keep in mind, however, that the bike paths are separate from the walking paths, so if your group wants to stay together, then two-wheel sightseeing is an all-or-none affair. There is also a kiddie train that putters through the park at set intervals, but save that for when exhaustion hits. There are kids' bikes and bikes with baby seats as well, but if you want them, try to get there well before lunch time or they could all be rented out.
That happened to me recently when I visited the park with my family, so our substitute exercise became a boat ride in the pond nearby. There are plenty of paddle boats on offer, but my kids begged to try a rowboat. I foolishly acquiesced. I can now say from experience that rowboats are not the best choice for two elementary school students and a landlubbing schlub like myself. If you know your way around a pair of oars, then please, don't let me stop you. If not, then it's quite possible that you will end up wet, vexed and floating in circles. The paddleboat is a much more elegant solution: simple steering, concentrated propulsion, and all that paddle-splash sprays away from you.