Hi Nick,

I have been to SGN. Its hot, messy and noisy. Some people love it...they even claim its the Asia they had been dreaming from afar. Others will just head to Mui Ne (beach, Kite surf), Dalat (lovers destination...) or Mekong River. Its a big city; you can take a half day trip to Cu Chi Tunnels.
Phuket? Head to Koh Phi Phi instead. Much more scenic...and you will avoid hordes of drunk idiots

I have never been to Colombo...but it seems a challenging destination.

Good luck