I'd be up for a riding trip to SL some time in early September, like weekend of Sept 7/8/9th or after. I've got plans every single weekend from now until then. Ugh. That's summer for ya'! It's a 4.5hr ride for me, so I'd have to spend the night someplace.
I was just at SL this past Thurs-Monday, but for a family trip. We had an awesome cottage near the Little Sable Point lighthouse. I didn't get to buggy the dunes, but it was by choice. I refuse to RENT one of those buggies simply b/c I could think of 50 things I could ADD to my buggy for the same price. LOL I've decided to ditch the "dollar" and I'm now measuring monetary units in terms of "buggy parts" from now on.

Next time, I'm bringing my buggies on the trip.
Every gas station and mart at SL sells the 10' dune flags. It's like $22 for a flag. Saturday & Sunday ORV dune passes (vouchers) go up for the taking the NIGHT BEFORE at 9pm (used to be 6pm). Really only an issue during holiday weekends (so I was told). One thing I didn't check into (and someone here probably knows) is which camping sites have "drive-off" privaleges or location so you don't have to have to get a parking voucher, too. There is a sound test. Must have a US Forestry approved tailpipe. Check the website for everything.
If the family wants to come along, there is plenty of beach to be had. There's a ton of state park public beach by the light house, and plenty more about 40-min north to Ludington State Park (miles of beach). There's the ~22mi Hart-Montegue bike trail which is more for exercise than for site-seeing. Plus there's a couple go cart places, ice cream place, and the Mac Woods dune scooter tour (yep, I took the tour with my 7mo daughter strapped to me on the Baby Bjorn, lol). Not much else to do there, really.