Even if you're not planning to climb Mt K, at least go white water rafting, go to the cultural village, go to hot springs, or go to Kinabalu Park/Mesilau and go jungle trekking a bit. I mean, you don't get these nature stuff in Singapore, why not do it there?
If you don't wanna go to that kind of nature, can go to the islands. Just note, many many MANY tourists nowadays.
Otherwise just spend time around town, eat and be merry.