I need a valid excuse to be leaving the house at 5am?

My girlfriend is coming to visit me from Canada (I am in England) and it is her first time in Europe. It is her birthday while she is here so I have booked for us to take the Eurostar to Paris and stay over night. So here is the problem. I don't want to tell her, I want to just be like 'come on, we're going out for the day!' and then see the look on her face as she realises we are getting on a train to Paris. But in order to optimise the trip I booked us on an early train, which will require us to be leaving the house at about 5:30am. So I need a valid reason why we would be leaving the house that early. Any suggestions?