shannon,
Don't forget you will loose 1 day flying to Europe.
one thing to remember is that every day your travel, you will be loosing time to do/see/experience things and consider at least 1/2 your day shot.
when it comes to food, everyplace has something to offer you, its will come down to what you will like or won't.
when i first went to london i gave it 4 full days. Munich i gave it 3 full days. I will be heading to Berlin this Sept and I'm giving it 3 full days too.
I gave Paris 4 full days the same as London since what i had planned to do/see/experience was penciling out that way and note that i didnt include any day trips outside of Paris either. Same with London, those 4 full days were in London.
You could add another place, but just remember it will take away some days from each of your places you want to go. If yorue willing to do that then go for it.
To me, i found that Paris was really cool, awesome and fun place. It was overwhelming, but in a good way and like Disneyland for Adults. So much to do/see/experience AND EAT!
Having been to Budapest and Prague last year i also found those places different but fun. I would easily go back too, but after i see some other places.
in my opinion, adding places that are off the beaten path will take more time for travel. Florence, Istanbul, Copenhagen and Barcelona are the ones i think of. Copenhagen maybe closer out of this group though. But if youre willing to chew up some of your to do/see time for longer travel times, then again go for it.
As far as travel goes, look at Seat61.com. link hereHe does a lot of the homework for you so all you need to do is to plug and chug A to B. Also, when it comes to buying train tickets, many times it pays to buy ASAP so you can save some $$$. sometimes you can just walkup and buy your ticket, but i would do some homework to find out when and where you can do that.
Just remember that the trains usually go from CITY CENTER TO CITY CENTER. No need to get additional transportation from the airport to the city since you are already in it! I find that i love the trains more and more due to that and that fact that youre able to wonder around more so than a bus. Its not as loud as a plane either.
Something to think about HOW you travel is to look at your path. the places you've chosen are more or less part of a circular path. Adding Paris will just bring the ends together.
happy trails.