Waterfalls - there is a whole selection of nice falls in the Pontneddfechan / Ystradfellte area in the SW corner of the BBNP, which would be a good photo option if the weather is dull. (TIC at Pontneddfechan for map/info leaflet). The Sgwd-yr-Eira is special in that you can walk behind the water but its quite a trek to get there.
Its a fairly easy walk to the top of Pen-y-Fan from Storey Arms on the A470 Brecon -> Merthyr main road - takes maybe an hour. You could be lucky. There are some good views of the central Brecon Beacons from the Brecon area, scout around and see what you can find.
On of my favourite spots is Llangors lake about 6 miles east of Brecon. The north side is the tourist hotspot with boat hire, cafe, toilets etc (and bleedin' speed boats....), but the south side near Llangasty is much quieter and prettier. (get there on minor roads or walk from the north side.) Access to the shore of the lake is otherwise quite difficult. (impossible, even!). The lake and mountains are overlooked from a minor road about 2 - 3miles to the E/NE of the lake between Mynydd Troed and Mynydd Llangors. You can walk from there to N or S.
Have a good day!