Thank you for your hospitality a13ean!
I will definitely go to some of the universities but I got an email from MIT that changed my plans. This could be a useful information for most of you (you probably already know):
If you get an offer of admission they partially fund your travel to the correspondent university. If you get several admission you can link several flights to see all the universities, and you will probably have to pay nothing. And they usually want you to go during the open house days.
So I guess I will wait a little bit to buy my tickets, but if I don't get an offer I will miss the chance to visit some nice cities