Hi there, I live at Tin Can Bay. The main beach at Rainbow is fine, just go a bit north of the patrolled area (not patrolled at the moment I don't think, maybe they have started again now) till you get some free area. Don't go south of it as you will be playing frogger with 4wd's.
Scope out the tides before you do anything, not sure how much room you would have to land at high tide. A lot of the beach got washed away 10 months ago or whenever that cyclone was off the coast, I can't remember.
If you have a 4wd check out Double Island lagoon like aden said. It's just on the northerly side of it. Nice flatwater, and exposed to the N/NE seabreeze. If you go south of it you get surf, but will only work if S/SE.
I personally wouldn't kite at Inskip near the bar, but I'm a wuss when it comes to bars. Unless you are pro, you will end up having your board taken out to sea I reckon :p on an outgoing tide.
If you want some flatwater come to Tin Can, best with a northerly seabreeze as it isn't too gusty. Southerlys are normally strong and gusty. Gusting up to 40/45kts yesterday

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Haven't had any experience with Carlo Point or the inside of Inskip, so can't really comment. Not sure how sheltered it is from the seabreeze. You will then have the long walks to the little bit of water left at low tide, like we do here at Tin Can. Can't really do much unless it's at least 3/4 full.
Hope some of this helps.