Llywn-on reservoir. It is the largest of the three reservoirs in the Taff Fawr valley, Brecon Beacons National Park.
I have now revisited all three of them this year with my parents. The last time I went to these places was about a decade ago, and whilst my life has changed considerably since then, these places are as nice as I remember them. Each are large volumes of flat and relatively calm water with a wee bit different surrounding.
Some phone photos of Llwyn-on Reservoir:
This one had a lot of forrestation around it. Whereas the first one I went to this year, the Beacons Reservoir (photos below), was mostly walled with a few interesting structures around it, and the Loops & Links path. The second one I do not have photos of, but it was similar to Llwyn-on.
So if you’re in town and want a nice place to a spend a few hours or so with ranging scenes, I recommend these three reservoirs and the area surrounding them!