Should I leave my SIM card here or what? I'm totally scared about those cheap and hidden charges they do for usage so I don't want to take any chances. I'm going there for 2 weeks so I don't wanna buy a prepaid SIM but is there anything else I can do? Use Wifi only or what?

I would disable automatic connection to T-Mobile Internet data service (Settings -> Internet Connections -> Connect Automatically -> Always Ask).

The T-Mobile US prepaid SIM in my N900 roams onto Rodgers in Canada (for voice and SMS) with no problems. Charges are higher, but not ridiculous. I suggest logging in to your account at t-mobile.com and checking the terms of your plan because its data roaming charges may well be ridiculous but you won't know for sure without checking.

Depending on where you are going... you can get a Windmobile prepaid SIM -- I think they do not charge for the SIM itself but you need to pay for an initial load -- their rates are very decent and cheaper than most. They are new so their coverage may not be as extensive. You can have your T-mobile phone forward to your Windmobile number if you need to so you dont miss any calls.