Great coastal scenery but a few steep hills.
Whalsey it is sometimes said is Shetland in miniature, and if you have only a limited time and want a short cycle ride this would be a good option. The harbour at Symbister combines modern buildings with restored 16th century dwellings. There is a sandy beach near the village of Sandwick in the south.
On Mainland, Voe is an attractive small village with a Norwegian feel to it: what looks remarkably like a fiord, with little wooden houses nestling around the pier. Brae five miles further north isn't quite so attractive.
The road on the southern section just east of Tresta rises to 114 metres from sea level but offers a great view of the two narrow peninsulas of Strom Ness and White Ness to the south.
This route is basically a northern and a southern loop starting from Voe, plus two possible additions at Lunna Ness and Whalsey. You might do one loop in the morning, the other in the afternoon and have lunch at Voe. The northern part includes the oil terminal at Sullom Voe. There will be a certain amount of traffic on the A970 and if you are cycling with children you might prefer to restrict yourself to Whalsey and the Lunna Ness peninsula.
This part of Mainland contains the largest area of high ground in Shetland and the mainland loops tend to be hilly at times. The B9071 south of Voe has steep hills but there are good views towards Muckle Roe.
Lunna Ness and Whalsey are much less hilly. Cycling out on the Lunna Ness peninsula will give good views of the southern tip of Yell. Whalsey would be a good option if you want a short cycle ride with a ferry trip.
Much of the time you will find yourself cycling by the coast. There is a lot of attractive scenery - don't be put off by the oil terminal. The narrow neck of land, interestingly called Mavis Grind, which seperates Sullom Voe from St Magnus Bay is sometimes described as seperating the Atlantic from the North Sea.