why not take the mini/short tour on your way home instead on your inbound trip? and as plenow said, theres a free tour you can avail of if your transit time in SIN is over a certain number of hours. If I am not mistaken however, flight has to be on SQ (but i may be wrong on this).
Having said that, you can happily spend 7 hours in SIN as there are many things to do..you can even go for a swim in their ambassador lounge (for a fee).
Not too sure what time you'll be arriving in SIN...but during peak hours and depending where you are...you can wait for a very, very long time for a taxi in the city centre (usually at around 6 PM)
If you want to take a short tour of SIN, i would suggest doing it after your BKK trip IMO