I have taken my daughter to St Andrews, Scotland every summer for the past 3 years. Prior to that, I'd not been in Europe since I was a teenager and not been in Japan since I was in my early 20s (darn job and being a grown up!). We'll explore a city or two then I drop her off at St Andrews.
So this is from a mom who does ALL the planning and has now done this 3 years by herself with a 13, then 14, then 15 year old.
First and foremost: W
here can everyone agree upon? You do not want to deal with an angry, sullen teenager (so not fun).
Once that's decided, what is your family's traveling style? All day on the go? Or some scheduled activities with some down time/free time?
Are you all morning people or more night owls with morning being a battle? My DD will decide upon one major and one minor thing she wants to do in a day (yes, only two must haves!) while I'm up and at 'em bright and early. I've been one of 5 people with the Crown Jewels at the Tower of London (2 being Beefeaters) and the only person in the room with the Book of Kells...all the while DD dreamed away. I was psyched to have the quiet, she was happy to sleep. Are y'all active? Do you overpack? Silly things but this will help narrow down your touring style if you go with a group (there is another thread titled "other options" which is very helpful with other groups and their styles). Since I know my daughter, although she would like the other teens on the trip, she would NOT want to get up and go go go. My DH, on the other hand, doesn't like a lot of other people, but wants everything to be seamless so we're doing ABD this Christmas and did another almost all inclusive company last year. Next summer DD, DH, DM and I will do Dover to Barcelona on
DCL with pre trip days in London and post trip time in Barcelona. Enough sleep in options for DD but enough hand holding for DH and time on our own for me! It helps to know who likes what and have the time to research and make it happen
Pacling-If you're on a cruise then your packing will be limited to the airline's restrictions and the size of your room (on my way back from Shannon this summer I could have had 3 bags when all I had was a carry on and a purse) whereas tours usually limit you to 1 piece of luggage and a small bag (purse/backpack/etc).
Food-on a cruise usually you (or the kids) can find more, um, American-ish food. On a scheduled tour, this is up to the tour and the restaurants contracted. On your own, you're limited to whatever you can find and/or have researched (I'll tell you my DH can and has found the worst pizza in Italy!).
Other kids/people to amuse your offspring so you don't have to- on a cruise in the summer if you're with DCL (and other family friendly lines) this shouldn't be a problem. ABD vacationistas can tell you the breakdown of ages/genders for specific trips (the reason we didn't do the Ireland ABD this summer was there were waaaay too many young children for my family...good to know before you pay a large amt of $$). On your own, unless you travel with others, doesn't have built in friends.
ETC- If you don't have one or it's out of date, apply for your passport as soon as you decide on a trip. They don't take too too long but you don't want to be holding your breath waiting for it before your trip-that is stress that can be avoided! Adapters...UK are different than Europe. They are very reasonably priced, even from the hotel's mini bar

o). Research the weather so you're packed appropriately. For our trip next summer I've been watching the weather in the cities I've been visiting and writing down the temps so next year I'll know what to expect (I'll also do a 10 day forecast before packing but this gives me an idea in case I need to order anything). I was able to buy really cold weather clothes (like long underwear, overcoats) for our ABD Holiday Danube/Prague trip on sale (whoo hoo) this spring. Of course with teenagers they tend to grow

Jet lag stinks so do everything you can to get to Europe as many days ahead of the tour as reasonable/feasible. I have crazy jet lag and reliably takes me 4-5 days to get over it, despite doing all the things recommended!
Finally, although there are not a lot of trip reports, those that you read on these boards are excellent! I read, re-read and almost memorize them. Cousin Orville, Mare1203, sayhello and Calypso726 are just a few of the outstanding photographers and reporters.