Greetings from Singapore! 🙂
Above suggestions are good, I’ve not done the night safari but hear it can be a bit hit or miss with regard to what you see. The zoo is ace, one of the best in the world I reckon, though our polar bear is getting a new enclosure so isn’t receiving visitors at present :). However, not sure I’d visit the zoo / night safari if I only had 48 hrs in the city.
You should definitely spend a day downtown, check out Chinatown & CBD then onto the Colonial district & quays area. Worth a trip on the Singapore flyer for the view – do it before it gets dark though. Top of Marina bay hotel is worth a look for the view too.
If you like seafood, as others have said you must try the chilli / black pepper crab here. I’d argue the best is Jumbo @ the East Coast Park about 15 mins out of the city. You can hire bikes and take a ride along the ECP, there’s also wakeboarding in a dedicated lake where you’re pulled by cables ala ski lifts here too if you’re into it.
Sentosa is an obvious choice as a tourist; it’s worth climbing up to Mt. Faber and then getting the cable car over for a quick look. The aquarium, as already mentioned, is superb. Universal studios is meant to be pretty rubbish unless you’ve got kids to entertain (I’ve not been).
Bukit Timah nature reserve is pretty unique, the only primary jungle left in Singapore complete with monkeys, monitor lizards and great bird life. Make sure you pack the mosquito repellent if you do choose to visit it.
I’d recommend trying to fit a boat trip over to Pulau Ubin to get a taste of what Singapore used to look, there’s also one of the two decent mountain bike trails on the island and the chance to hire bikes to ride on it if you want to get some biking in. It’s a 10 min boat trip from Changi village which is very close to the airport so you could try and fit it in on the way to/from the airport.
The food is great here, try and find some murtabak while you’re here – it’s one of my personal fave foods of Singapore.
I could go on, but you do only have 48hrs in our fair city so will stop at that. If you want to do anything specific, or want any other questions answering drop me an email (address in profile).