Sitting just a short bit off Kailua Beach, Flat Island is a frequent destination for locals and tourists looking to get off the beach and into the ocean. Dozens of people a day (at the least) take their kayak or longboard to the breaks just off the right edge. Officially called Popoia Bird Sanctuary, it's nicknamed Flat Island for obvious reasons. The pancake-flat outcropping has a sandy beach to park your craft and some small sandy coves around the islands shores. You are not allowed into the interior of the island for the protection of the seabirds, but the tide pools, fishing, snorkeling and watching the surfers keeps you plenty busy on the outskirts of Popoia.
The shallow reefs on the right side of the island tend to have at least some small waves, and the paddle is only a quarter mile each way. Eastern swells are less common than northern and southern ones on O'ahu, so bigger days are less frequent here in the protected bay. Launch your kayak or longboard from the beach just left of the boat ramp and not from the ramp itself. It's pretty much a straight shot with few high coral heads to worry about. In Kailua Bay you don't always have the best visibility, but it is common to see honu and smaller reef fish swimming around. The beach on Flat Island is not very big and can get crowded; it's more of a spot to visit and set up camp for the day. This is a much easier and safer journey than the Mokulua Islands, but is obviously a lot less dynamic. It's a good starting point for people wanting to try some interesting ocean adventures without pushing the envelope too hard.
There are kayak rentals near Kalapawai Market outside of the beach park. Boat ramp parking is limited, so the beach park has a larger lot if needed. They all get crowded, so early and/or on weekdays is the prime time. Check the weather, bring some water, and keep a limit on your sun exposure through lotion, hats and long sleeve water shirts.