Seconded on kayaks and canoes being fun...or even an inflatable boat of any kind. My grandparents had an inflatable rowboat and we kids loved it when we visited.
Is there a place on the shore to set up a fire, hammock, rope swing, and/or slackline? any of those could be fun (as long as the bug spray works, sounds like).
Also, it doesn't sound like you do, but if you did want to get your kids into water skiing, my parents' friends pulled it off with a larger jetski (it had a place built in to tie the rope to...I've only gone once so I don't know the technical terminology). Obviously still expensive and not frugal in the way the other suggestions are, but it's probably cheaper than a boat, and up to you to decide if it's worth it.