Best beach imo: Tien beach, 2nd place Monkey beach when it's high tide. But you can explore the whole of Koh Larn in 1 day with a motorbike if you wanted. It's a small island and easy to get around on a scooter. Also it's very quiet now because of no Chinese so even the main beach looks ok. ????
Accomodations, I have no idea, I only go for daytrips. ????
And yes, Use Bolt if you can. Easy and cheap for transportation.
Have fun!