The good news is that the 2 towns are only bout 20 minutes apart and are an easy drive.
Ok, so if you're doing one day at CP, and one day at Kalahari, that leaves you 2-3 days of doing "other" stuff. I would consider doing one full day at Put-in-Bay, then maybe another day split between Kelley's Island and Marblehead and/or Catawba Island. PiB has a party reputation, but during the day, there's tons of family friendly activities. Kelley's is a great family island, but it's not chock full of things to do. You can find kayaking at PiB, and if you want a fishing charter, do a google search for charters out of Port Clinton, Huron, Sandusky, etc. I don't know of any offhand though. You can rent jet ski's at Lyman Harbor.
Other places to look into:
Downtown Sandusky for cute little shops and parks.
East Fork State Park near Port Clinton for a great beach and swimming.
Marblehead lighthouse near Port Clinton to wade on the rocks and take in some great views.
There's some great waterfront restaurants on the Islands, but on the mainland you're limited. Oh, and there's no "boardwalk" anywhere. You could take a walk along the waterline in downtown Sandusky, or go for a walk on the Pier in Huron. Either of the TGIFridays locations offer waterfront dining. There's also Damons and Lyman Harbor in Sandusky. My favorite thing to do is to go get ice cream at Toft's Dairy in Sandusky, and then drive to the Jackson St Pier in Sandusky to eat it and watch the boats.
I'm not sure of the price, but you can check into Sawmill Creek resort in Huron. It offers restaurant (one on the water), a private beach, huge indoor and outdoor swimming pools, cute little shops, and a golf course. If you want a taste of 1950's style family motels, check into The Maples motel in Sandusky or the Plantation Motel in Huron.
Keep in mind, it is PRIME BUG SEASON on the lake! Muffleheads (smaller, fly/mosquito looking) and Mayflies (archaic looking, long wings, long "tail") are or will be all over the place when you're there. They will be worse at some locations than others. Keep in mind that both bugs are harmless, don't bite, and are just annoying. Also, be mindful of ferry boat times if you go over to the islands.
www.shoresandislands.com is a great resource.
Please let us know how your trip goes, or if you have any other questions!