If it were me, I would start with AutoEurope. I've been renting from them in Europe for over 20 years, and I've never had any problems. They act as a consolidator, and so you go to their website, key in your information, dates of departure and arrival, and destinations. Then they will show you a list of car rental companies that have cars for where you want to pick up. They will show you the prices, all of the relevant information, and then you choose which car and which company you want. I like that Auto Europe maintains control over it, so if you ever have any problems, you can contact them. They're always concerned about making sure that you're happy with your car rental company of choice.
You will need to let the car rental company know what country you will be traveling through with the car, as you may need to get stickers for border crossings or permission, or there may be other restrictions involved. Once I rented a car in Germany and drove to to Austria, and the car already had a vignette sticker on it, so we didn't need to do anything. But that might not always be the case.
I'm assuming you know this, but when you drive from city to city, parking is always going to be an issue, and sometimes driving in the cities themselves. Some cities now have congestion fees that they enforce for cars that are driving in the city during certain hours. But again, that's something you can research and find out ahead of time.
If you're interested, AutoEurope has a page that gives you specific driving information for every country in Europe, so you might want to check that out as well. Scroll down until you get to the country listings: https://www.autoeurope.com/driving-information
Edit: Russ is correct that there will almost certainly be a drop-off fee for dropping off the car in a different place, especially a different country. The price can vary depending on the rental car company and the locations.
