it was actually really easy. we bought our tickets online and had them already when the gate opened. we had gotten a cab companies number the night before at the POR front desk and called so there was a cab waiting for us the next morning. we were dropped off and after 9 hours in the harry potter corner of the park, we called the cabbie directly and he showed up where he dropped us off and took us back to disney.
renting a car would have been a little cheaper but then we'd have had to drive(and probably get lost), pay to park, and would not have been there at gate open. i got some great pics of just my kid in what looks like a totally empty hogsmeade. totally doing the same thing next trip too.