You do not say what aircraft you plan to use. Hope it is / you are happy with short strips ! They are defintely a challenge and not all flat either, especially Skerries and Papa Stour.
Agree with foregoing posts but also bear in mind that there is limited or no parking room on some of the strips so it is essential IMHO to liaise with Loganair so you do not block a runway and cause a scheduled flight to return to base without landing !
Fair Isle has parking space though and Tingwall is a good friendly base from which to explore. The outer islands are gems to visit. Enjoy !