Here are some really easy ones (grades 3 – 5, yet they sound impressive):
J. S. Bach – Little prelude BWV 999 – Play this prestissimo and mouths will open.
Benda – Sonatina no. 3 in A minor - In my opinion the nicest of his one-movement sonatinas – Fast arpeggios shared between hands and impressive crossing of hands.
Beethoven – Six Ecossaises – Rhythmic and uplifting, yet incredibly easy. Kissin often plays it (superbly) as an encore.
Clementi – “Grand Waltz” – the glissandi are the impressive bit. If you want to be really impressive, play them as superfast scales.
Dacquin – “Le Coucou” – Lots of running passages.
Mozart – Variations on “Ah je vous dirais Maman”. Several of the variations are very impressive.
Albert Pieczonka – Tarantella – energetic, rhythmic, easy and very impressive.
Now for some more difficult ones, but still pretty manageable for common mortals grades 6 – 8+):
Scarlatti – There are around 250 virtuoso sonatas that are manageable by most students. Have a look here for my favourites:
https://www.pianoforum.net/cgi-bin/yabb/YaBB.cgi?board=repo;action=display;num=1077145772J. S. Bach – Giga from Partita no. 1. The crossing hands are spectacular, and you will needs lots of confidence, but it is not really that difficult.
Chopin – Polonaise Op. 40 no. 1 in A – The score is thick with chords, but this is actually pretty easy to play and pretty impressive to watch.
Handel – Suite no. 5 in E major – Both the prelude and the “air with variations” (The harmonious blacksmith) are suitably impressive.
Mendelssohn – “Spinning song” – This is Song without words op.67 no. 4 – Pretty impressive – sounds much more difficult than it is – Rubinstein used to play it as an encore.
Mendelssohn – Song without words op. 53 no. 3 – ultra fast arpeggio figurations on the LH, this reminds one of Chopin’s revolutionary study for which it is a good preparation.
Mendelssohn – Song without words op. 102 no. 4 – a tarantella with ultra fast staccato on the RH. Impressive yet far easier than it sounds.
Mendelssohn – Cappriccio in A minor, op. 33 no. 1 – Almost never performed, this is a wonderful piece. After a slow, lyrical adagio, all hell breaks loose. Mendelssohn at his best.
Paderewsky – Minuet op. 14 no. 1 – Lots of fast runs all over the keyboard. Paderewsky is said to have grown to hate it.
Prokofiev – Prelude Op. 12 no. 7 (nice melody on the left hand, with a fast broken chord figuration on the right, plus some glissandi).
Debussy – Arabesque no. 1 – Lovely piece.
Debussy – Dr. Gradus ad Parnassum – Much easier than it sounds.
Grieg – Holberg suite – especially the first movement (“prelude”).
Chabrier - Bourée Fantasque. A bit more difficult than the previous ones, but well worth the effort. Exhilarating and uplifiting
Carl Maria von Weber – Perpetuum mobile – lots of impressive, fast fingerwork.
Villa Lobos – O cravo brigou com a rosa (The clove quarrelled with the rose) from “Cirandas” (no. 4) – very virtuosistic beginning and end with a lyrical middle bit – based on a Brazilian Nursery rhyme, another of Rubinstein’s encores.
This should keep you busy for a while.
Best wishes,
Bernhard.