How much more pre planning?

Make sure every driver has an IDP:

http://driventoit.blogspot.com/

I have no experience booking either of those experiences, but my general rule of thumb is that the more specialized an activity is, the earlier it needs to be booked. I'd also consider how disappointed I'd be if I waited too late. On both counts I'd be most worried about finding a cooking class at the last minute. The walking tour could fill up, too, but you could partially replicate that yourself by reading Rick's guidebook. There may be multiple companies offering walking tours, and some of them may even be of the just-show-up type. Sometimes the local tourist office has walking-tour brochures as well, though I don't know whether Florence has anything along those lines.

You may wish to post your plan. One option visiting Florence is to park at the Figline Valdarno station and take the train. There are others. On the car rental make sure you have picked a reputable agency and have zero-deductible collision.

Have not done their Florence tours, but have done their Venice tours and they were excellent.

https://www.florence-freewalkingtour.com

Edit, using the Waze app will help in avoiding ZTLs. Check to see if you have to add this on to the regular Waze.

If you take a "free" walking tour, keep in mind that Rick reports the guides must turn over a fee to the company for each client. I believe the amount is usually something like 3 or 3.50 euros per person. So don't try to get by with a minimal tip like 5 euros. That would be really unfair to the guide.

Totally agree w/acraven, we usually tip 10-15€.
P/P.

Small group tours often sell out. I wouldn't worry about getting a general 2-3 hour walking tour. Some of the sights in Florence (esp. Academia, Uffizi) need to be reserved in advance.