Greece is all Mountains with no guardrails, not just the islands. Santorini is beautiful, but expensive and stuffed with tourists big time.I would not worry about the guard rails in the least. it's not that steep really. My least favorite place in the islands, but if you have never been its worth going.
The beaches on Santorini are terrible, relatively, forget them. The best thing to do besides the sunset, is go to the excavations at Akrotiri. You can get a Bus tour which is worthwhile, in Thira, right up from the bus station. It will take you around to many of the coolest places you'd have a tough time finding any other way. Hotels charge a ton to pick you up at the ferry. I'd fly in if you can afford it, or take the public bus to the town and walk to the hotel if you are on the ferry.. The ferry ride is pretty long. like 7 or 8 hours. the high speeds are shorter, but only operate when the water is smooth, on rough days they do not go.Email me if you want more information. And get rick steve's Athens and the blue guide. There is way, way more to see than you have time.
Athens is not the most gorgeous city, there is lot's of graffiti.. I assume you have a hotel already. if it's close to the subway, that's good. there are lot's, and lot's of things to do. Do not rent a car in Athens.!
there are tourist restaurants and non tourist restaurants. the tourist ones are in the Agora mostly.My favorite is Kefanio, on Loukianou st, not in the agora. Up the hill from the embassy's, towards the funicular. Excellent food and not touristy or expensive. The hotel will help you and definitely get the blue guide. it's worth the money. it's at amazon. Greece is steep, be ready for that.