Kailua town itself is small enough to cover on foot or by bike — most rentals are within ten minutes' walk of the beach. The town has good bike lanes on the main streets, and rental shops (including beach gear) cluster around Kailua Road and Hahani Street.
For the surrounding sights — Lanikai, the Pali Lookout, Olomana — you will want a car. The local bus service (TheBus routes 56, 57, 67, 70) connects the town centre with the beach, neighbouring Waimanalo, and Kāneʻohe but is infrequent. Taxis and ride-share work but can be slow to arrive.