Singapore is a very popular tourist and business destination with a lot to offer the visitor, so the list of attractions is a long one.
Among the things for which Singapore is famous are innovations in design and science, so it's not surprising that the top attractions include museums and centres dedicated to these. You should certainly visit the ArtScience Museum with its constantly changing displays and huge collection. In addition, the museum has become famous for the building itself, and its spectacular setting in Marina Bay Sands, an attraction in its own right.
You may also like to visit the Red Dot Design Museum, a building famous both for its bright red exterior and its unusual collection of exhibits and shows dedicated to design, new tecnology, advertising, gadgets etc.
For further ideas, it's a good idea to browse Tripadvisor, which lists the wonderful National Orchid Garden as its top attraction but has information on hundreds more.