Everyone has given you great advice, nevertheless I'll suggest some not-that-difficult pieces (someone even suggested La Campanella o.O) but that can be really fun to play since we don't know your level. For Octaves, the most important thing, I think, is the position of your wrist and how relaxed your hand is. The Entertainer has fun octaves (or sixths? I don't actually remember), Chopin op 28 no. 12 is also fine, and maybe Rachmaninoff 1st prelude c# minor (that's a challenging one, but again, we don't know your level). Bach is awesome for technique too... Like awesome, and Moszkowski études will also give you really fine skills. For arpeggios, Kabalewski's 5th study should be fine. Pretty much anything, if played well, will improve your playing. Chopin has some beautiful stuff. His nocturnes are beautiful. I would suggest op 55 no. 1. That will give you balance, arpeggios, light touch, speed and a fine use of the pedal.