I know that Lomster is smart enough to have already thought of this, but please don't try to learn anything new. There have been some really great and imaginative ideas for effects on this thread, but I don't think there's time for you to learn even a new presentation of an old trick in the few days left.
The advice I'd offer (not that I am a pro) is to consider the two different times you'll be performing, and the nature of the events.
In the reception, people might not have hands free (if there are nibbles as well as drink), and you certainly can't rely on any kind of table-based tricks. You're probably expected to be an ice breaker and get people talking and relaxed.
By the time you get to the table-hopping bit, people are quite possibly going to be somewhat drunk (or very drunk) and you might have the option of using the table (if you don't mind dodging wine stains and bits of food). It's probably best not to rely on it. Keep the plots simple (not that you would do anything complicated in either bit). I think your ambitious dog card routine might be good here, along with sponge balls or bunnies.
Good luck, and yes, it certainly doesn't help when people move the goalposts almost at the last minute.
