It might be best to wait until the airlines publish their winter 07-08 timetables before making any definite decisions. It would seem to make sense to choose a destination where there are evening flights in both directions. (This is so that you can travel out on the day of your wedding and get the maximum amount of time at your destination before returning).
If Ryanair keep the same basic timetable structure that they have now, Barcelona would be an excellent choice. There's an outbound flight to Girona at 1800. This lands at 2115. The bus into Barcelona takes a little over an hour (although you might want to take a taxi,as it's a special occasion), so you should be at your hotel before midnight.
The return flight leaves Girona at 2200 (so you'd be able to enjoy most of the day in Barcelona), arriving back at 2330.
Barcelona is a wonderful city to visit at any time of the year:
http://www.barcelonaturisme.com/?go=jN7uAYLx/o IJaWVQCb4ZU93ygJVYLOpRQdvyIue/wAW6Ddhyhal0Y8Jy bdczumN4RSbo3CRL94sJBt7u1Kc=
Chris