I had no idea Trinidad and Singapore were islands, but I actually can't believe I forgot Cuba and Taiwan when I got Niue, Aruba, Cook Islands Faroe Islands and Curacao.
That's a tricky one! They've built a causeway across the strait separating Singapore from Malaysia, but there was a bridge over the water at the northern end. However, that might have been filled in - and if so, does that make it a continuous causeway, and if so, does a man-made causeway mean that Singapore is no longer an island country? Is there anyone in Singapore who knows?