I'd agree 100% with the Sonatine. I prepared that for my ARCT a few years ago, and I loved learning it the entire way through.
The coordination in the first movement was one of those 'holy crap, how do I play this so fast and quiet!!' parts, and alone helped SO much with letting me get a completely different tone on the piano.
The second movement has a lot of jazzy chords, big stretches, and also requires a refined touch to voice them properly. Great for identifying complex harmonies (I kind of relate this to an easier version of 'Le Gibet').
The third movement has a lot of fast technical passages; it's so tightly knit structurally, but offers such a variety of things to play (change of meters, very interesting chords, fun fast passages to name a few) that as you work on it, it's impossible to be bored.
Jeux d'Eau right of the bat might be too tough - it's got some cadenzas that are harder than some of Liszt's (I was working on Un Sospiro for the same exam, and tried Jeux d'Eau for fun, and those damned passages were MUCH tougher to read and coordinate at the time). It also has some really strange harmony that you might not be used to right away. The technique in some of the quiet passages is extreme (the Sonatine has very similar moments, but again is generally easier, so it would give you a chance to get used to his writing).
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Try comparing it to someone whose first Debussy they want to play is 'Feux d'Artifice'. They MAY have played pieces that tough, but if they haven't played any easier Debussy pieces before, they won't have a clue what to expect technically or musically. If they had a few of the easier Preludes done first, maybe the Arabesques, or a piece from Children's corner, they will be exposed to his style a bit more.