DH and I went several years ago, and the only thing we booked ahead of time besides accommodations was a rental car.
I love Banff. I can honestly say it’s the most beautiful place I’ve ever seen.
When we went, we basically just drove from Banff up the main highway all the way to Jasper (stayed in Jasper for a couple nights) and back, checking out the various lakes and sites in between. We did day trips from Banff most days, then after a few days made the drive to Jasper. We did the bus that drives out onto the glacier, which was super cool, but just bought tickets when we got there that day.
We went 10 yrs ago with then-young kids. If you PM me your email, I'll share my trip report from that time (my trip reports have evolved and improved since then but should still have some useful info).