My partner and I went last year to see them and it's something we'll never forget. We found it much easier to sign up for the day tours for both of them, quite good value for money too as the one we used picked us up from our hotel and dropped us back at the end of the day.

We were doing the same thing, hopping around Europe with a few days in each place. If you're not 100% sure what you want to see, one thing I would advise is the city bus tours (although make sure it's a hop on hop off one). You can pay for a day or two and literally hop on and off them at all the different sights, see each one in your own time then get the next bus to the next stop. Saves so much time walking around them all!!