Friends of the Uffizi

Good idea! You can do multiple short visits (I believe the Florence card limits to one visit per museum)

It works great, allowing you to make spur of the moment visits multiple times. However, I don't think you can just walk in and join. I suggest you check their website. Make sure you get the website for the Italian organization, not its US-based counterpart. Each time I've joined, they ask you to fax in an application form before you show up, and then when you go to the museum, you go into an office to pick up the card. There's not really a membership desk like you see at other museums. So I would suggest closely following the directions on the website.

My daughter and I were in Florence this past November/December and we joined Amici degli Uffizi. We did just walk in and ask for the " Amici degli Uffizi" desk and joined. You must have your passports as ID. But, if you prefer, you can join online. One of the benefit is that at some of the museums, you can skip the line with the "Amici" membership; and that can save you a lot of time. It is well worth it! We went to a museum at least 5 times a week. There are many places included in this. There is no limit on how often you go to any of the included sites. Do not miss the Medici Chapel; it has tombs designed by Michelangelo and is quite beautiful. Also, the Pitti Palace and the adjoining Boboli Gardens are included and have so much to see.
Enjoy and Buon Viaggio!