2. Ferry from Aberdeen
There are a number of ferry options, and it really depends where you are starting from as to which is most suitable for you.
If you are starting from Aberdeen it would make most sense to take the Northlink ferry from here. There is a ferry on four days per week during the summer and it departs at 1700, arriving into Kirkwall at 2345 (6 ¾ hour duration). On the way home there is again only one choice of departure, with the 2345 ferry arriving back into Aberdeen at 0700 the next day.
Whilst this is quite a long journey the ferry is very comfortable, and well-equipped with restaurants, bars and lounges. You can also take your car on the ferry, which is convenient for travelling around in Orkney.
The price for a return trip for 2 adults plus a standard car is around £350. If you want to add a cabin (perhaps for the way back only) you can, with prices starting from £93 per person for a bed in a twin cabin (each way).
Incidentally, if you wish to visit Shetland too this is the only way to get a ferry from the mainland to there. Once it drops off in Kirkwall it carries on up to Sumburgh, arriving at 0730.