Guitarists of BT - please help!!!

Ok - here's the deal.  My sister is having a medium-sized, casual outdoor wedding.  I am playing guitar in her ceremony.  She is super laid back (think anti-bridezilla) about the whole thing and wants me to decide what to play.  I think I have figured out what I'm playing during the processional, but I am completely blanking when it comes to the recessional.  

I have been playing for many years, but at a rather sporadic, intermediate level.  I won't impress any real musicians in the crowd, but I have a few weeks to learn a not-too-difficult song and make it sound ok.  I am the only "musician" in the ceremony and I will just be playing, not singing.

So, any ideas for a recessional song?  There isn't a wedding party (which is one reason why I ended up in this role) so I imagine the recessional will be very short and I will probably only end up doing part of the song.  I am hoping to find something celebratory and/or humorous that will be fairly recognizable.

Also, I don't play in bands or do gigs or anything like that, so any advice on playing in a situation like this is welcome!!  I do plan on practicing my songs in front of others before the ceremony.