Need help making itinerary 5 days Thailand
 

Family of 3 with teen ager reaching Bangkok mid March at 10 AM. Planning to spend one day in Bangkok. Need to decide itinerary for remaining 4 days.
Plan to fly back from Phuket with late night flight (departing around mid night) which will give us total 5 days including the day we arrive and the day we leave.

Where to stay and which places to visit in Bangkok for one day? I have Hilton status so get free breakfast or should I choose some other hotel chain? We can easily take public transport or if recommended then we can hire Taxi.

Can we have recommendations for remaining 4 days too? Should we go to nice resort/private beaches in southern Thailand? Should we do island hopping? Should we visit elephant sanctuary?

We like sight seeing - fast paced and then come back and relax in resorts/beaches, enjoy good Thai food.

Thanks in advance.