When you are going to go on a cruise the Cruise Company will give you literature usually a brochure with all the places of interest that give you lots of information, it will also give you the tours they will do with timings. After you read this, you can contact Tour companies in Shetland and arrange your own trip, but find out what you want to see first, then compare prices.
Asking on a Forum is good for information, but you may only get the views of what others want to see and do.
You will find tour guides here
Alternatively you can take a bus to Scalloway or Jarlshof
Doing a google search, these websites may help you...
For ideas of what to do, follow the linkjs on this map.
Jarlshof and Sumburgh Head make a good trip. Also allow time to explore Lerwick and Clickimin Broch.
Scalloway has a ruined castle and also a small museum about the Shetland Bus (if you are intrrested in WW2)
Make sure you book well in advance of your visit as Shetland isn't very big and tours will get booked up quickly if there is a cruise ship. This also applies to taxis.