Start with 5 minutes of warming-up-impro containing these elements: 5-fingers, scales (also in thirds/sixths, chromatic), arpeggioz, octaves, double notes. Or do some of the Brahms exersheeyats, they work very well.
5/10 minutes a polyphonic piece (doesn’t have to be Bach)
10/15 minutes a big Etude in :wim: tempo
rest of the time a part of a Sonata or anything you’re working on
I’m hardly a good pianist so this is not a recommendation, but the way I do it, anyway, is to skip dedicated exercises altogether and draw them from the music I play instead. The first thing I do when approaching a new piece is to play it through and extract the passages I know will take the longest for me to master, and then those are my exercises.
Also, when sitting down at the piano I invariably begin similar to how Erwin recommends above. Improvisation is too fancy a name, it’s just free spinning across arpeggios, scales, thirds or wherever I end up. More often than not it’s also all I do!
It depends, but if I is maintaining a prog or tiz in da last stages of rectal prep, I would prax all da uncomfortable parts, den look at a few tempo transitions. Also make sure dat da first few seconds of da piece is gud
Dere is always sum spots n positions, especially in da Chopets 25/10 n 11 (where every detail must be present fo da rec) dat require mo prax, n I jus start wiz dose spots - being careful not to over drill machnically but always look fo a new way of thinking in rapin dem
already buzy TEACHAH zchedule ? 
