Chidorigafuchi Park is a lovely place to take a breather. Enjoy a picnic lunch, amble around the grounds or just lie on the cool grass and close your eyes for a while.
You're spoiled for sightseeing choice when spending time in Chiyoda. Chidorigafuchi Park is a great place to start here, but there are many other fine places to appreciate. Tokyo Skytree, Tokyo Tower, and Tokyo Imperial Palace are near the recreation and green space and and will cap off your day nicely.