We shall have from 6.30am until 3.30pm between flights in February.
It must be worth going into Singapore - where we've been before. Any suggestions on how to use this time enjoyably.

You can take a 2 hour, and free, city tour right from the terminal and will not even have to go through customs and immigration. I believe you remain on the bus except for 1 or 2 stops.

The airport hotel also offers very clean shower facilities and a spa. It is in the transit area so, again, no need to go through the formalities.

You can also, as you have been before, just take the SMRT train from the airport and 'wander around', maybe head somewhere downtown.
http://www.visitsingapore.com/

If your baggage is checked through that is fine, if however you are connecting say to a budget carrier then note that you often cannot check in until several hours before, but there are places to store your baggage.

As Nofly says, there are excellent facilities in the transit area, the transit hotel, you book by blocks of hours, also great showers, also foot massage, cinema etc.

If yuo are connecting from one long haul to another decide if you want to be worn out so you sleep on that connecting flight, if the connection is only a short one then perhaps take the chance for a few hours sleep at the transit hotel
http://www.airport-hotel.com.sg/

Me, I am a Singaporean, if you are having a transfer, you may go to the East West Line to Tanah Merah Station(EW4) for City to have a city tour strolling across the Singapore River!

Or visiting the Changi Airport is not bad too!

That free city tour is really great. It is only available for people on a layover.