I think it depends on how far you really want to travel. Sandusky does have the waterparks but it's probably another 1:15-30 from Cleveland.
Vermilion is a small town on the lake, right now they still have the Great Lakes & Maritime museum and it's nice to walk around downtown. (Garth Brooks & Trisha Yearwood just bought a home south of there also.)
Cleveland has lots to offer also. The Flats can be fun, more geared toward adult fun. Love the Metroparks. Tower City also. You can walk around in Cleveland Heights, they have their own little Italy and Coventry (think hippie stuff).
Oberlin is another small town in the area. They have an art musuem, college and a lot of little shops.
As for the RRHOF, I wouldn't be surprised if you kids know more than what you think. We have friends that took their two sons (13, 11) last year for spring break. I think they had a display on the Stooges, kids knew who it was.

My own kids would have a field day in there (17, 14, 12). I think it depends on you, if you show them a lot of music, different genres, they will know and appreciate what they are seeing. (Do they still have Jimmy Page's dragon suit on display? I know they rotate things.)
In the past they have offered something called rock and roller coaster deals. For a certain amount, you had tickets to the Rock Hall and Cedar Point. It might have included hotel also. It was a great deal from what I remember. Not sure if they are still offering this though. Something to keep in mind for a later date.
Maybe the Cav's are in town that weekend also. It might be worth looking into.
Also can see if the IX center has anything going on during that time.
If you get a chance Bubba Q's in Avon. Mmmmm. Old Prague in Vermilion. Marconi's in Huron (Vermilion and Huron both on the way to Sandusky).