Unlike Scott, I've never had a huge bill when I've returned from a trip. I always turn off data roaming in the airport in the US before departure.
I use the phone only for emergencies. My kids will text me a few times (they watch our business for us while we are gone)
Last trip I found several spots to use the wi-fi, so I was able to check email and surf the web a bit.
I'm on vacation, so I don't use the phone that much to check on business. If I really need the internet, you can still find a few internet cafes, or the hotel will have a free or inexpensive access to the web, so I use that instead.
If you are only using it for music and travel apps you will OK, as long as roaming is off, and you don't send or receive a bunch of text messages. Tell your friends to NOT text you during this time - you get billed for incoming text messages as well as sending them.
When I returned from my trip last May, my bill was about $10 more then normal - that's all.
Why do you need to use your phone on vacation? Are you calling ahead to hotels? Checking in with other travelers? I never purchase the international plan. I was told long ago by a customer service person that if I keep my calls around 20 min, that is about the same as paying for the plan for one month...so I keep track of my time (but then again - I don't make calls on the phone - in Europe it's for emergencies only)
Your trip sounds fantastic! Have a great time...