Checking my options for international travel

I have an iPhone 4, with iOS 5.0.1. I am leaving to go to Japan for three months in May and my carrier in the US is AT&T. My phone will probably be used as a GPS in case I get lost, something to be used to call my grandparents who live in Japan, and to possibly text/call my friends briefly in the states.

My question is then, what would be my best option when i travel this summer? Should I try and unlock my phone with a gevey SIM (I refuse to jailbreak my phone)? Should I buy the international plan before I leave? Or should I buy a pre-paid phone when I land in Tokyo and use it instead? Is it possible that there is another solution?

Thanks for your advice!

Edit: Clarity.

Edit 2: In case my response gets buried, the firmware is 04.11.08