If you truly feel you will not need to use your phone for calls or texts, and can manage all your day to day needs over Wi-Fi, you can leave your phone on Airplane Mode and just turn on Wi-Fi. If you feel you might need a phone to call in an emergency, you can simply add the free “International Voice Roaming" and / or the free “International Data Roaming” add on. Buying any of the Travel Add-ons only nets you a savings on your roaming fees, it does not change anything about the ability of the phone to roam. Without the Voice / Text / Data Add-on, you can still roam at pay-per-use prices. Have a look at this thread for some additional info.
I have often travelled without a plan, and survived with expenses of about $2.00 for the few SMS I send, and about $10 or less in calls.