When you sign up for the tour, you will receive your first and last hotel so you can add extra nights there if you like.
The one advance reservation that was recommended was for Neuschwanstein castle. I did not see the interior, but after hearing other tour members describe it, I would if I had to do it over again. Book the first English tour of the day to avoid crowds later in the day.
We arrived in Chamonix around noon and many of us made reservations on the way for the Aiguille du Midi cable car as it was a nice clear day. So, it was 1.5 days there. The second day I took the train and cable car to see the glacier.
Two full days in the Lauterbrunnen Valley.
If you get an early start, you can see a lot in one day. And since you’re going in May/June, that will be an early sunrise time. So, you will have time to get out and explore before breakfast if you want.
Oh I think reservations were also recommended for Eagles Nest from Salzburg. A few tour members did that.
I arrived in Salzburg a few days early and made advance reservations for the Bob’s Sound of Music tour.
https://www.bobstours.com/tours/sound-of-music-public/
I also had reservations for the Panorama Tour trip to Hallstatt.
https://www.panoramatours.com/en/salzburg/tour/hallstatt-tour-945/
I trusted my instincts and was not discouraged by those who say you need to spend the night to enjoy it. We were the first tour bus that arrived during a weekday and it was a very pleasant few hours. If it’s a choice between not seeing a place and seeing a place for a few hours, I will often go with the day trip.
If you think of any questions during you planning, I’m happy to help if I can.