I used to use Changi Airport a lot a few years ago, so am a bit out of date. FE is the expert, I an sure.
However a few points:
1. There is a very good left luggage dept, I have left stuff for a week or more, so you need not take all of it to the hotel.
2. Taxis are not too expensive, I cannot remenber a bus, but would not bother.
3. I would try to spend 2 - 3 days rather than 1.
4. Eating out is great - try the Seafood Centre on Esat Coast road, or one of the many Hawker centres.
5. There is a "Transit Hotel" inside the check in zone - useful to get your head down for a few hours if flight times are awkward.
6. The Zoo is the best I have visited, Jurong Bird park good, as are the Botanical Gardens just North of Orchard Road (a main shopping area),.
7. What are your dates? The Divali festival is in November - a great time to be there.
8. Wish it were me going!! BTW Singapore Airlines are the very best (if you could afford them)