Chanonry Point on the Black Isle is great for dolphin watching.
On the west coast, Ardmair camp site, a few miles north of Ullapool, is excellent and you are not far from some interesting geology.
Ullapool Camping & Caravan park - Ardmair Point Holiday Centre
Good fish and chips close to the ferry terminal.
Gairloch further down the west coast has two campsites, one (Gairloch Caravan Park) - small and quiet - in the centre of the village, the other (Sands Caravan & Camping) about 4 miles away is a large dunes site.
Millcroft Hotel in Gairloch is good, but if you are in the area, the Badachro Inn, across the bay, is a must -
Badachro Inn Web Site
Lots of places on the west coast run wildlife boat trips and most beaches are good for a bit of beachcombing.
End of September is probably still OK, but I suggest you check any campsite, etc first to check they are still open.
Although some councils in Scotland discourage folks from stopping overnight in laybys or by the roadside, it is not a problem in the most of the Highlands.