It's usually full, but we've made reservations a week or two in advance and been OK. I think it's been getting harder, I seem to recall a recent trip (that I didn't go on but was included in the emails) that they went at 11AM because it was the only time available. (ask me about chocolate overload at 11AM. *GAGS*)

There's a chocolate trolley trip, it's expensive ($80 each) but might be just the thing. I had proposed it for next weekend for the chocolate girls, but the price was a bit offputting for right now.
http://www.trolleytours.com/boston/chocolate-tour-boston-restaurants.asp

I can't think of anything else, except call the hotel back and see if there's a cancellation list you can get on. :(