The Visitor Centre at Calanais (for the stones) on the Isle of Lewis is now open on Monday to Saturday (not Sunday, as you might expect on Lewis)- https://calanais.org/
You are asked to pre-book, probably because of the limited parking facilities- 33 general spaces, 6 EV spaces, 4 accessible spaces and 10 bike racks, no camper vans or mobile homes.
The admission price until 31 August for adults is £13.13 (£12.50 + 5% VAT) then £15 after the VAT reduction scheme ends.
The pre-booking seems to be due to the parking, because there is plenty of availability despite 5 big cruise ships being at Stornoway this week.
Out of hours (including Sunday) access will be by a path at the side of the visitor centre, but there will be no toilet facilities at those times.