Leis are usually redily available at the airport, but you will most likely have to buy them yourself. Package tours offer lei greetings, but if you booked this on your own, maybe running off the plane first and getting a bunch of leis might do the trick. Otherwise, you could arrange with the hotel to greet you with leis upon check-in.

The rental car might be useful if you want to venture away from Honolulu. Tours are available on the big busses, and some places like Ala moana have shuttle services to the major hotels/shuttle stops in waikiki. You can take "the bus" around the island (public transport) but this is for the adventurous type. http://www.thebus.org/

Good luck, enjoy HI, and have a spam musubi!