I can recommend a couple:
Macphun have a whole suite of apps/extensions (most but not all work from within Photos)
- Noisless cleans up the 'noise' from low light, high ISO pictures
- Tonality gives a pile more options than the (very lacklustre) 'Filters' in Photos
- Intensify lets you add special effects to make your photos pop
DxO also have an extension that corrects lens artefact (chromatic aberration and pin cushioning) as well as one click enhancements, noise reduction etc. The trick with this extension is that you have to do it before any other edits.