Well whatever you do, don't promise him these will be your last ones. I did that back in May, and now the opportunity has come up to add another one to my herd. So I'm in the same boat as you.
I will tell you what I have done so far though. I cleaned out the extra cage I have and put it next to the other two. I think if he gets used to seeing it there he won't be too surprised if there is an occupant some time in the next two weeks.
The kids keep asking me if I told *dad* yet. When the time is right!
I like the Christmas surprise idea and not enough room at the shelter.
Good luck to you in whatever you choose to do!