Hi
If you want to see the Pyrenees, meaning high mountains sceneries, without a long detour. I would recommend south of Pau :
- Pic du Midi d'Ossau in Ossau valley. If you are ready to walk, go up at least to Bious Artigues lake (short walk) or longer to Ayous lakes. In this valley, if you don't want to walk, you can take the small train of Artouste which will bring you up to Artouste lake and also offers, on the way, good views to Pic du Midi d'Ossau. You can spend the night in Laruns village.
- The second option could be Lescun in Aspe valley. A small village in a very scenery mountains. Going down to San Sebastian, you don't need to go back to Pau and can make short visit to Navarrenx and Sauveterre de Bearn.
If you're ready to longer detour, Cirque de Gavarnie is one of the most famous place in Pyrenees (south of Lourdes).
If you're ready to see only lower mountains, there are plenty options in Basque country : up to Rhune mountain by train (just above see shore), Baigorry valley a bit at the east or Saint Jean Pied de Port (famous because it's on the "Way"h.