Easy. Depends on what your tastes are but you could, for example, take an overnight sleeper to Trang and go to the Trang islands (Ko Kradan, Ko Lipe or Ko Muk) or Ko Lipe for a bit of beach and snorkelling.

Then either take a train or a bus to Penang maybe, bus to KL (only takes 5 hours and Malaysian buses are quite luxurious) then bus to Singapore. There's now a direct ferry from Ko Lipe to Langkawi as well if you prefer (not sure if it runs in low season). Very easy to arrange overland transport once you're here though sleeper berths on trains are best booked a couple of days in advance at least (particularly if there's a full moon part on Ko Phangan or it's a Thai public holiday.) You just go to the train station and get a ticket, or a travel agent in BKK can get it for you for a small commission if you prefer.

__________________
“Not all those who wander are lost.” Tolkien.
Life is not measured by the number of breaths we take, but by the moments that take our breath away.