The Churchill Barriers can get closed any time of year, so South Ronaldsay and Burray are out as places to stay. The only usual passenger-only ferry is the one to North Hoy landing at Moaness. But vehicles for other ferries have to be booked the day before and the weather can stop them going too - you wouldn't want to be stuck off Mainland on the day of your return home!. So definitely stay on Orkney Mainland