Adagio and Allegro by Handel is a little more challenging. Lots of notes, very baroque.
Sonata IV by Bach is similar, maybe a little more difficult.
Contest Caprice by Ostransky is a good one.
Bach cello suites are a good choice. Unaccompanied. Pretty tough.
Bozza Improvisation et Caprice would be good for a college-bound senior.
Villa-Lobos Fantasia is really hard, for a kid who wants to major in performance.