So Adriaan and I spent 3 weeks in Europe back in 2016. We were originally going to do a cruise too and decided the time in each city would have been too limited so ended up deciding on which cities we wanted to see and do 3-4 days in each over the 3 weeks.
We did Venice, Rome, Barcelona, South of France (Nimes / Marseille), Amsterdam and Paris. We changed cities every 3-4 days and it was a LOT. If we had to do it again (and we definitely will someday), we'd stay longer in fewer cities and would skip Paris entirely... we hated Paris, but that could be because at the end of 3 weeks and changing cities every few days, we were too tired to really enjoy it. If it had been the first city, would we have enjoyed it more? We'll never know because we won't go back haha
Romajo gave you lots of great info on currencies... yay for that! I was gonna say that I thought most would be euros aside from the British pound, but I didn't think about the Swiss Frank.
We booked airbnbs for all of our stays. They were insanely beautiful but we weren't really thinking budget at the time, so I'm no help there really. Though I think you can still get good deals on airbnbs in europe and they have so much space and personality compared to hotel rooms which I love.
I will say we didn't "plan" a lot of activities throughout our trip because Europe is just so so sooo different than the U.S. it was wonderful to just wander and experience and be in the moment.
Traveling between cities is a breeze! We flew into Venice, took a train to Rome, flew to Barcelona, train to Nimes, plane to Amsterdam and then train to Paris and flew home from Paris. EasyJet / RyanAir are your friend for budget flights around Europe. They're no frill, but soo soooo cheap! Book all of your flights / trains / etc on the carrier's websites directly if you can.
You're going to have a wonderful time! If you want any restaurant recommendations or any standout spots from any of our overlapping cities, just lmk! I'd be happy to share <3
I think our next big BIG trip is going to be Tokyo / Kyoto next year sometime once Emily graduates from Uni. We were supposed to go for her high school grad but Covid ruined that plan.