Every normal practice day I usually start off with stuff like scales, tech, reading, ear training etc as I find it easier to keep my motivation up if I get that kind of stuff done early.
On a "rest" day I skip all that. Maybe I will just jam chords and riffs for an hour (which could end up being a song), or else maybe learn a new song or play through my repertoire.
Then after that I'll probably mess around with a looper and end up soloing for a while.
Basically the fun stuff, but it keeps you ticking over.