we have reviews of the Mandarin here- you've picked a good one!
Singapore- I tried to get your hotel for when we are going in July but couldn't. It looks good.
Things to see and do in 2 days
Singapore Sling in Raffles( and I can recommend Doc Chengs there for dinner )
China Town and Little India are musts.
The zoo- either during the day or at night
Bird Park
if you like shopping see if you can squeeze in a couple of hours at least- some great bargains there
Sentosa- bit hit and miss but there are some great things to do there if you have enough time. My opinion anyway- the first time we went we had 3 days and didn't go.
The Battle Box- if you are interested in WW2.
Chaingi prison- we went before it was due to be shifted and it was brilliant but not sure if the shift has happened and if it is still good to see.
I will have missed out things here- thats just off the top of my head

Jess
The Mandarin Oriental in KL is great!!! I would stay there again anytime. It definitely had the WOW factor when we walked in The lobby is just beautiful.
The breakfast has a huge selection of stuff.

I am sure that you'll love it.

Mandarin Oriental WOW hotel although the standards rooms are a bit small...great views of park ond over KL. Lovely infinity pool over looking KLCC. Nice location and as Zabka says great breakfasts.

Stayed at the MO in april this year a great hotel only a couple of very slight negatives, the room lighting is a bit dark (seems to be the trend with most of the hotels in KL) and the bathroom doors are a bit weird.
Overall great location, great staff, Pool area and spa highly recommended (eating dinner by the pool in the evening doesn't feel like you are in the middle of KL).