I am pretty boring when I go to Bangkok as I keep returning to the same place. I can't see past the Landmark hotel. Great position and good pool. However, first time we went was to the Bangkok Marriott Resort and Spa and this might be ideal for you. It can hard to realise you are in Bangkok as the area around it is very green and it has a super pool area. You take the hotel's ferry boat across the river to the Skytrain link.
Marriott website
I also loved this hotel but preferred the location of the Landmark
Landmark

Where are you staying in KS?
By the way- great idea to spend time in BKK at the start and end

This year will be the first time for my family to spend a holiday in Thailand. We have booked to stay for 10 nights on Koh Samui and 1 night and 2 nights in Bangkok either side of the holiday. Bungalow booked on Samui any tips for somewhere to stay in Bangkok would be useful, me hubby and 14yr old son. One other thing left to book is our flights which we will be doing in the near future. Any comments on things to do on Samui would be wonderful

Have a look at the Pullman.http://www.pullmanbangkokkingpower.com/ Quite a new hotel, only a year old and is number 3 in tripadvisor at the moment. They are offering great deals just now aswell. The location is perhaps not as touristy as Landmark, but has more of a local thai feel about it apparently. Very close to sky train which is all you really need in Bangkok. Also its only a 30 min drive from the airport which might be good for you as your only staying a short time. For your flights, Thai Airways, Eva Air and especially Etihad have some good deals just now