I don't want to do a ton of pre-planning, however I would like to go
on the trip armed with an idea of what to do daily. Also, do you
suggest making reservations at tourist attractions beforehand?
This is where a guidebook is invaluable! What to see? What are you interested in!? If I hadn't read one of your previous posts then I would have sent you a list of fabulous churches full of artwork and/or history to take your pick from...except that you wondered in that post what you could see besides churches. So, it's best for you to spend some time browsing what's available and marking the things that spark your curiosity lest your trip become someone else's. :O)
Yes, there are some attractions which require reservations/pre-purchased tickets to skip long ticket lines, and they tend be the most-visited sites (e.g. Colosseum and Vatican Museums in Rome; Uffizi in Florence, etc.) and a few which have limited access, such as the Galleria Borghese in Rome.
Aside from just being "armed" with an idea of what to do, it's really helpful to understand WHY some of the choices are especially important/interesting before you get there.