This is a worthwhile day trip. To get there from central Singapore, you need to either take a bus or taxi out to Changi jetty on the northeast side of the main island, near the airport. There you catch a bumboat (ferry) for a 15-minute ride out to the island.
Pulau Ubin is largely undeveloped and supposedly a lot like what the rest of Singapore used to be like before all the development. When you get off the ferry, there's an information desk where you can get a map of the island, as well as a bunch of bike rental places and simple restaurants and snack shops. I chose to walk around for a couple of hours, and found it to be relaxing and beautiful. I did get a bit lost, but finding my way back was no problem. I had some really great Hongqiao noodles at one of the restaurants as well. If you're tired of the city, head here for a break.