I’d strongly recommend the National Gallery for those who appreciate art and architecture. Each work is accompanied with a write-up in English, but even if you’re not interested in reading, there’s plenty of art to visually take in.
Your ticket should allow you access to multiple exhibitions, including those on Singapore’s art history, Southeast Asian art through the last few decades, and a couple more that are regularly rotated throughout the year. Tickets are also free for locals, which is awesome! Do look up the exhibitions prior to coming down, in case some are closed in preparation for the next installation.
The gallery isn’t too crowded - I went down on a Sunday afternoon and had lots of space to myself. If you intend on carefully going through pretty much every art piece, I suggest you dedicate a full day here (or more). Otherwise, an afternoon should be sufficient for a simple walk-through of the various exhibitions.