When you think fishing, you think Montana or Washington or Tennessee, but not Cleveland. Yet the Rocky River Reservation was rated one of the top 150 fishing spots in America by Field and Stream magazine. I can personally attest to its quality as I have actually caught fish here, showing that it's a veritable haven for fish.
The stream tends to be a little crowded on weekends, but if you can skip work on a fine April morning, you'll find plenty of open stretches where it's just you and a hook and whatever is swimming along.
If you're more of a land lubber, you can find great running on the bridle trails or the main all-purpose trail. It's mostly flat, but you can jaunt up one of the entrances if you need some hills.
This effect makes it surprisingly secluded from the rest of the city around it. When you descend into the stream's valley, the only signs of civilization are the occasional houses that overlook the river at various places (I always hope they aren't watching my poor excuse for fishing).
They have a lot of special events, too; the Settlers Celebration in late September is always enjoyable old timey fun.