It is fucking expensive machan. Anyways travel requirements change time to time
https://www.singaporeair.com/en_UK/lk/travel-info/covid-19/
Also when I went back in March you were only allowed to use SG Airlines if you wanted to use the vaccinated Travel lane.
What to do in SG --> Universal Studios is there. Sentosa Island is there. ofcourse shit load of shopping can be done. Shit load of food is there to try out. But for everything you need money. there is close to 50% increase in prices if you consider SG and SL. For instance a Mc Meal that costs 800LKR here will cost 1000+LKR in SG.
Commutting too is quite costly but pretty easy and neat. You will pay something like 300LKR to go distances you would pay say 30LKR in SL