Wow. I can't thank you guys enough for your in depth help and advice, we really are first timers to all of this so its fantastic to get advice from people who know the area as well as you both do.
AZBuck, I think you're right about leaving Ohio & Memphis out this time - a good excuse to come back! We're not so keen on the theme park side of things so we probably won't visit Florida, but thanks so much for your advice RE: the motel's - thats exactly what we were hoping, that we can just 'rock up and find a room'! Great advice regarding using the metro in Washington - Foy, I can't believe how expensive it was to keep the car at the hotel!! We will definately be using the metro option.

Wow -Foy, your descriptions have made me feel even more excited about our trip- its sounds magnificent! We had never even thought of hiring bikes and using the cycle lane but this is such a great tip - it sounds amazing.

Thanks again to you both - will be printing these replies off and taking them with us!
Rebecca