I'd upload some Bitcoin to an Instawllet and send them the link.
As you say, that way you can reclaim it if they never take it. I don't like the idea of the recipient only having 14 days to claim bitcoinmail, but then, the service is free so they need to pay for hosting somehow.
Even better. I would include links to merchants where they could spend the coin. Best of all, you could do it at the last minute so you don't have to plan ahead like when you mail cards.