If you are an immense rock fan, and sit through all the movies, you can spend a full day at the Rock Hall. If you aren't rock fans, and just want to see it, a half day is plenty.
Read the reviews at Cleveland "Things to Do" and you'll get a better idea of how much time you need.
The U.S.S. Cod is a 10-minute walk east of the Rock Hall, takes about one hour, and is a highly acclaimed attraction by TA reviewers. It's the only remaining unmodified U.S. WWII fleet submarine. Visitors enter and exit through the original hatches and ladders, just as the members of the "silent service" did over a half century ago, and can imagine the experience of WWII underwater combat.
The Steamship Mather, docked next to the Rock Hall, also amazes most visitors. It's the retired flagship of an ore carrier fleet, over two football fields in length.
Also, while in Canton, you're a short distance from the Holmes County Amish country, the center of the largest Amish population in the country.
The Cleveland zoo will take four hours at a minimum. In the summer, save the excellent Rainforest exhibit and other enclosed buildings for the mid-day heat. Animals in the outdoor exhibits, with the apparent exception of the African elephants based on reports, will be more active in the morning or later afternoon.
A one-day visit to Cedar Point is a risk given weather and even mechanical concerns. Even if the weather is perfect and lines relatively short, coaster enthusiasts likely would be disappointed with a single day in the park.
Also, check out Presque Isle and the Waldameer amusement park, just west of Erie, PA. In August, Presque Isle offers one of the very best freshwater swimming experiences on Lake Erie, where the water temperature can get well above 70 degrees in August.
You're journey could benefit from extra days. Just visiting what you have on the plate will be extremely rushed and tiring, and involve long drives.
Reportedly you can buy discounted Cedar Point tickets online if you are AAA members.