COD is right, only Oban and Stornoway have regular supplies of avgas.
Lots of places to go in a 172, although if it's with 4 POB you'll need care at some of them. Oban and Mull are musts. Plockton's great. Tiree, Barra, Islay, Campbeltown all plenty runway (or sand!) and well worth the visit. Gigha, Coll, Colonsay, Bute all possible but do your performance calcs before you set out. Benbecula's the only way to get to the wonderful beaches on South Uist but it's a jobsworth airfield in a desolate location.
Have fun!
NS