So there are no constraints on time periods?
Ravel's Ondine is a water piece, so what if you chose two of the 'nature' Debussy Preludes. My prof went on about this for like 15 minutes when I was picking some to play (I chose 'Vent dans la Plaine', 'Colline d'Anacapri' and 'Ce Q'ua vu le Vent d'Ouest).
Out of those three, I honestly find le Vent d'Ouest the least strenuous to perform. Vent dans la Plaine has those awful jumps in the middle, and Collines d'Anacapri has loads of little intricacies combined with jumps. Vent d'Ouest is just more bombastic, and I find that it's generally more grounded than the other two.
I dunno, Brouillards, Feuilles Mortes, des Pas sur le Neige, Ondine, and the three I mentioned have something to do with nature in one way or another, so maybe two that fit well together?
Or just pick one longer piece by Debussy: Pagodes, Reflets dans l'Eau, Soiree dans Granade, Hommage a Rameau, ... anything in the images or Estampes pretty much.
Or do a more contemporary French Piece. Francois Morel's 2nd etude de sonorite is pretty cool.