Originally Posted by Thunderroad

There's some sort of a complimentary Singapore city tour for folks in your situation. I believe it's run by the airport rather than by SQ, but am not sure. Sorry I don't know more about it.

Regardless, you should definitely take advantage of The Private Room, the lounge exclusively for SQ first class passengers. Very good service, ambiance, bubbly and meals available there. You can access it at any point in your connection. Just make sure you have your onward F boarding pass in hand. (You should get it when you check in for your J flight.) I don't know the hours, so you might check on that, but I'd assume it's open during your connection time. I'd also assume it has showers to freshen up, though I haven't used them myself.

Also, give yourself some time just to wander around the SIN airport itself if you've never been there before. Quite the scene. In particular, the Butterfly Garden is worth a visit.

Finally, if you decide you want to lie down for a rest during the long connection, there are airport hotels inside (i.e., you don't have to pass through security) that I believe offer reasonably priced rooms in four-hour blocks. Guests often need to book them well in advance, however.