Originally Posted by turnleftbrighteyes

You should definitely rent a (or if traveling by ferry, considering bringing your own) car. There are buses, but your mobility would be very limited and they are not so frequent. Details about the inter-island ferries are here. You can call ahead to book a place for your car, but last summer (at the height of tourist season) we never had a problem not getting onto the ferry, so in general it doesn't seem necessary.

Your flight options are with Loganair or, apparently, after September 1 with Eastern, which will be codesharing with Flybe. I would recommend having good travel insurance to cover any pre-paid bookings in case your flights are delayed, as is not infrequently the case with flights to and from Sumburgh.

I'll be taking the ferry to Aberdeen in September, so can report back any notable experiences then.