First of all congratulations!! I would suggest u enjoy the summer off and make the most of it :) as once u get started.... u wont be having a summer!! Wen i accepted my offer i emailed my first supervisor and he said to get away from uni as far as possible lol :-) If you want to prepare urself for the start in september u can email ur supervisor for reading list to get ur head around the topic. Enjoy the summer 8-)