I would recommend some sort of round trip between Ballynahinch, Downpatrick and Newcastle. Ballynahinch (quite big sub-depot of NC) still operates Leopards to Belfast; the 17:00 B'hinch to Belfast is nearly always a Leopard, most likely 204 (VOI 204). It's also easy to get Leopards between Downpatrick and Newcastle; there are two or three buses that leave DP around 16:00. You could change in Clough to go back towards Ballynahinch if you didn't have time to make it to Newcastle.An idea might be: 14:15 from Belfast to Ballynahinch (occasionally Leopard operated), get the next bus to DP usually around 15:30 (would be jam-packed with schoolgirls though) and then on towards Clough or Newcastle. Most of the buses from Ballynahinch to Downpatrick at this time are NC buses and then work from DP to NC so that wouldn't give you much layover time in DP I suppose. Then from Castlewellan, Newcastle or Clough back to Ballynahinch in time for the 17:00 to Belfast.
Maybe that's getting a bit late in the day. Belfast to Lisburn and then to Saintfield (26B Rural Transport service, all services DP operated and quite often with Leopards) and on to Downpatrick would be an interesting route.
That's just a few random ideas for a day out I had; feel free to ask any more questions.