I would have said Top-Cwm Dwr through trip in OFD as thats a superb through trip but more than four hours, so not of use to you...
So... Cwm Dwr - fair bit of crawling involved to get to something worthwhile (IMHO) but if you plan on going back to OFD at a future date its useful for route finding next time! I would recommend a trip in and out Top Entrance - plenty of big stuff, some nice formations and plenty of passages to wander around in (get lost in) within a short distance of the entrance..
Finding Cwm Dwr is very easy as its in a quarry very close to Penwyllt, Top Entrance less so but not too bad - I'm sure someone at SWCC would give you an idea of where it was if in doubt.
Pant Mawr - looks good, never done, but would like to - I do believe its a looong walk though!
Little Neath is also a superb wet sporty trip for dry days with a few short holes nearby.
Access is an issue at present in the Little Neath area - check out the
http://www.cambriancavingcouncil.org.uk/ for info if you are considering anything in that area.