It's expensive! I've been there three times over the last seven years - last time in November last year. This last time the high cost was evident even more than on previous occasions as the Singapore dollar is much stronger against sterling than it was. First couple of times it was about $3:£1, now it's 2:1.
It's a very well organised place but is becoming increasingly sterile as they knock down the older buildings and stick up the new stuff. The metro is superb and will get you anywhere, pretty much. If you are going to be there more than a few days get an EZ pass, it's their version of an Oyster card and is worth picking up. BUT only if you are there for long enough.
There isn't actually all that much to do other than eat and drink. The Zoo is very good, as are the Botanical Gardens. They have a stunning orchid section there.
Take a small umbrella/packable waterproof as it can piss down in any "season". It's near the equator and and doesn't really do seasons but there are times when the rain is more prevalent.
There are some things you can do cheaply and others you can't.
If you are so inclined you can keep food costs down by eating at fast food outlets, food courts and hawker places. That's what I normally do. There is just about any food available including some of the best Indian available anywhere.
What you can't do is drink booze, smoke or stay in decent hotels cheaply, you can only temper the expense. You can find cheap hotels but I wouldn't want to stay in them. It's worth looking at Chinatown for more reasonable prices, you can also eat very well there too. Little India, I'd go and eat there but not sleep.
There's also Sentosa Island if you are theme park inclined. I'm not so haven't been but it is evidently quite popular.
Oh yeah, it's really **** hot and humid!
Also, if you are a smoker you can't bring any in. It's illegal. I found out by accident after bringing in 200 that I picked up at Dubai before the connecting flight. I didn't get stopped by anybody, which was odd as I was carrying a rectangular box that could only really be a 200 carton in a Dubai Duty Free free plastic bag. Just as well as I subsequently discovered there is a decidedly stiff fine for it.
TBH, if you are there for more than 2-3days you'll find yourself rapidly running out of anything new to do.