If you are relaxed, "wing-it" travelers, then there are only two things you need to decide:
1) Is there anything you MUST see?
2) Is there anything you need a reservation for?
For Rome, if you think there is any possibility you will want to go to the Borghese Gallery, it needs to be booked in advance. The Colosseum does not, but use the tips in Rick's guidebook (purchasing ticket at the Palentine Hill) to avoid the line.
I would still pack a guidebook just because it is so useful, and read the information in it before I went. Other than that, just take it easy and have fun.
As for walking, Venice is pretty compact and walkable (no other option besides boats anyway). I found Rome pretty walkable as well, but my feet were definitely sore some days and we made good use of the metros. You might consider a combination--ride the metro out away from your B&B and then walk back. I liked the map my hotel had just fine, but there are also excellent recommendations on this site, just do a search.
The thing to remember about navigating in Rome is that the city is arranged in 'circles', and that landmarks really help you navigate around. For Venice--eh, just expect to be lost. That's part of the charm.