If you are into WW2 history there are two must do things here for you in Singapore. The first is the Changi Chapel museum, which is likely the more well known of the two, out near Changi prison and Changi airport.
The other is the “BattleBox” which is a bunker built into the hill that Fort Canning was located on. It’s a similar concept to the Churchill war museum in London. This bunker was were the decision was made to surrender to the Japanese in Feb 1942.
The only downside of the museum is that you can’t take pictures inside. Trust me though, it’s awesome. Both museums dispel many myths about the fall of Singapore, the main on about the guns pointing the wrong way. Before the invasion the BEF were well aware the invasion would come from the north.
The BattleBox can easily be accessed from Fort Canning Station, an uphill walk but there are outdoor escalators. The parklands around Fort Canning are beautiful and worth a look even if you are not interested in WW2. There is even a small wading pool on the south east side, so take your swimmers!
Changi chapel is harder to get to, there is a bus, but I’d recommend getting a cab. Both museums tell the same essential story, but with the chapel focused on what happened after the fall with the BattleBox focused on the events leading up to it.
The BattleBox is only open a few days a week (Friday, Saturday and Sunday). If you can only do one – tough decision, but I’d pick the BattleBox
If you are looking for lunch there is a fantastic little restaurant next door.