Many car rental agencies are at the airport so even if you haven't booked you can book after arriving at the airport
. The drive to downtown is quite easy and the route is quite straightforward. To find a street in downtown that will allow you to drive in the direction you want to go is not that easy and so you might find yourself driving around for a while. If you know the city (not necessarily too well) it can take from 30 minutes to over 1 hour (in rush hour).