I just returned from the same area, with many of the same goals, in April. Lascaux IV was stupendous--such an amazing recreation in a wonderful building with engaging exhibits once you exit the cave replica. Well worth the admission charge.
Agree w/the poster upthread on Rouffignac---the little train takes you 2 km underground past images of diverse animals (mostly mammoths), cave bear scratches & wallows, and ends at the great ceiling covered in many, many animals. The cave is perched on the hillside, has outdoor public restrooms, and might make for a quick picnic stop if you timed it right. The tickets aren't sold in advance, and you either turn up early in the AM for the first tours, or just after lunch to get a spot in the afternoon tours (what we did).
In Les Eyzies, we had a peaceful, relaxing (& inexpensive) lunch at "La Dinette & Cie", an almost-riverside simple creperie near the bridge & a canoe rental place. Smoked trout, local beer & wine, friendly service that didn't mind our limping attempts to speak french. Board of specials, too. It's far enough from the main drag that the hordes miss it, and you can take a nice walk down the river path before or after lunch.
Put La Roque St. Cristophe on your list, too: while the exhibits focus on medieval history & occupation, the actual SITE is stupendous. FIve (or more) levels of slots and overhangs on a towering cliff face over the Vezere river make it easy to understand how rock shelters were favored by Cro Magnon. Then drive up the river valley to St Leon sur Vezere & have lunch at Restaurant de la Poste. Or if you're really on a budget, drop in at the town grocery, which has an array of savory & sweet pastries brought in from a bakery, and take your picnic lunch down to the riverside behind the church.
