Your guide will also give you suggestions based on where you end up with your group activity. I realize this does not help with advance planning though!
About 2 months out from your trip (maybe a bit longer) you will get the General Europe guidebook which you can use for finding activities in each place. This may be available in your local library as well.
The things I saw that I paid for on my own were a Vivaldi concert in Venice (maybe 22 €?)and I visited the Capitoline Museum on my own in Rome(maybe 10 €). Just realized I forgot our guide loosely organized a wine tasting in Bacharach as well. I think it was 22 € per table with 4 at each table so 5.5 € or so each.
I paid to enter the Baptistry and Duomo in Florence so that was about 10 €.
People who hike in Cinque Terre have to buy a trail ticket (maybe 5 or 6 €? Didnt hike so not sure). Others took the ferry from town to town or rode the train.
In Switzerland I hiked back to the hotel from Truemmelbach Falls along the public path...free, lol!
Many got tickets ahead of time to go up in the Eiffel Tower. Others planned an excursion out to Versailles. I would not recommend missing any of the Paris activities for a day trip. If side trips are important I suggest staying a few days longer.
Pre trip in Amsterdam I did the Dutch Resistance Museum, maybe 8 €. The schedule indicated the possibility of enough free time in Amsterdam after the Rijks Museum to possibly see the Van Gogh Museum which is right nearby but there was really not time for it.
The places where you will not have to plan activities as the planned stuff will probably take all day are Amsterdam, Bacharach, Rothenberg, Reutte, & Beaune.