As people have suggested checking out Devil's Lake SP, it's a great park IF you can get in. That's a VERY popular park, you would've needed to have reservations already. You can roll the dice and get there around 3:00 pm and wait in line for "cancelations and no-shows". Reservations need to be made by January or it's going to be tough. Same goes for Mirror Lake SP. We made reservations back in February and got one of the last spots available for late August and we could only get middle of the week, NO weekends available. You can go online at Reserve America and check out availability.
It's an awesome area, good luck with your adventure.
Dan