I completed the Phuket Sandbox on 14 August. I took the Sandbox Express bus for 1500 to BKK and stayed at a hotel near the airport that night. I have since moved on to Nonthaburi.
Other than the bus, there were flight and taxi options. You can rent a car if you can find one. If nothing else, take a taxi to the bridge then hop over to the other side to grab another taxi. It's not necessarily easy or historically cheap to leave the island now but it is possible. I know several people that have gone as far as Chiang Mai from Phuket recently.
As for the Sandbox stay... Many businesses are still closed but more are opening each day. You should have little trouble finding something to eat or keep yourself entertained. There's lots to do and less people to get in your way now.