A full day tour to Phi Phi including a one night stay is in my mind a top 1,
in reality you use 2 days, departure from hotel 8 in morning and return back 1800 hour next day.
But as you already have planned 2 days in Bangkok and a visit to Samui,
then Phi Phi is maybe not the time.
In general I could stay on Phuket forever - but have to stay here in Denmark and work
I lived for one year in Phuket som years back, did not have problems using my time.
No mather if you visit Bangkok, Samui or Phuket - try to go out run with Hash House Harriers. They meet once a week and run/walk 5-10km, you will get new friends and see something special. Search on google for location and meeting place.