What can I do at Changi Airport?

EDIT: WOAH! Didn't expect to get so many comments and suggestions from all you wonderful people! I've got some great ideas and I think I know how I'll be spending time at Changi! I'd love to take some time and reply to you all. Maybe I'll do that on the plane haha. For now, I'll thank you guys in this edit :D

Hi guys, I'm flying with Singapore Airlines to Australia, the day after tomorrow with a stop at the Changi Airport. Initially I had only a 3hr layover. But SQ suddenly changed my trip and now I have almost 6hr of layover. I'm traveling alone and I was wondering what I could do in the airport? Also what currency/payment methods can I use at the Changi to buy food, duty free items etc. (since my layover was short I hadn't planned to do anything here and didn't arrange for SGD cash and it's too late to get them now)