Or better yet, a user definable default using a combination of text and data formats to include data taken from the file such as start time or lat/lon, elevation, length, i.e. any of the fields already available in the file list.
Cryptic examples:
Bike_%StartYYYY_MM_dd_Bike_%StartHHMM.gpx
to get Bike_2009_January_30_13-21.gpx
and so on
Hike_YYYY_MM_%StartHH:MM.gpx
Run_YY_M_%Start_HH:MM.gpx
I think you should get the idea.