I used to visit Sandön every summer in my youth. It's possible to walk around the island in one day. My friend did, but my knees didn't think I should. And I don't normally have a problem with my knees. It's very taxing to walk along the beaches, mostly loose sand and partly stone. I recommend that you take it in two laps. There are paths crossing the island so you can walk along the beach around half the island and back across the island and take the other half another day. That way you also get to see it all, both the beaches and the inland. The inland paths are also on sand ground, there's no solid ground on the island, but more compact, easy to walk.
Camping is only allowed at the camping site by the lighthouse village. I'm not sure if it's allowed to sleep on the ground without a tent outside the campground, but definitely not with a tent. Neither can you use any kind of fire outside the campground (no stove, no smoking...). So taking day hikes is sensible in that respect too.
In my day we went with a small fishing boat from Fårö (north Gotland), but that was before the commercial ferry existed. I don't know if that's still an option. I guess not. It sure made it more of an adventure!
It was also possible to get food sent out from a shop in Fårösund, but I doubt that works now. Then the lighthouse was stilled manned and so was the radar base, so KA (Swedish Coastal Artillery) took the food with them on their regular tours and kindly delivered it to the camp ground and we could then keep it in an earth cellar at the village. Not everyone knew about this. It was extremely satisfying to fry fresh lamb chops and drink milk when everyone else was having freeze dried crap and porrige. Those were the days! 😁
Have a good time! It's a wonderful place. I'm sure you'll love it.
