Pre-paid plans for visiting the US

Hi

I travel to the US a few times a year (personal travel, not business) and was told that T-Mobile's pre-paid daily plans are a better option than roaming packages.

Does anyone know:

1. If I buy a SIM card for my unlocked phone at the store, can they activate it for me there? Or will I need to do that at a computer at my hotel?
2. How long can I keep the SIM active on the pre-paid plan? If I don't care about keeping the phone number (it will mainly be for data and a few outgoing calls), is there a way to reactivate it if it goes inactive, rather than buying another SIM?
3. Is there a minimum top-up amount I have to put on the account, and how long does it stay in the account before the money expires?

Thanks