I too lived in Singapore for three years in the late 60s/early 70s. DH and I met there so have some very fond memories. We have been back - three or four times - but it has changed beyond all recognition.
The Botanical Gardens were always lovely, but I understand the Gardens by the Bay are amazing.
Take a boat trip and view the sights from the water.
Street food is wonderful.
Go shopping in C K Tang.
Get some made to measure clothes, choosing your own fabric and style - they will be ready in 24 hours.
When we lived there Raffles was quite run down, so didn't bother with it, but went for lunch on one of our return trips, and had a Singapore sling in the Long Bar.
Go to Mount Faber and view the fantastic seascape from the top, then take the cable car to Sentosa island, or cross by ferry.
Enjoy the colonial style of the government buildings, court buildings etc.
I am thinking of visiting late DH's cousins in Australia at some time so will probably have another stopover, but sadly this time on my own, but hope to have happy memories.