Here’s a few things I’ve messed with when i just wanted to doodle around and not repeat a tune.
jumping from 1st to 3rd position & back
Build licks that incorporate sliding 3rds & 5ths
Run scales in as many 2,3,4 note combos as you can invent
Simple arpeggios work great for building different skills…run through an entire scale doing them. Example: if you go up the G scale the second appegio you would play will be an Am, then Bm, C, D…and so on keeping all notes within the G scale. you can run them up, down or both for each passage.
I found it best to start with 3 note arpeggios and add a note each time I get comfortable with what I’m doing.
Circular licks: usually a short number of notes that come back ot the first note.
Run them up & down any (every) scale or chord progression you care to.
Find three places on the neck ot play each lick.
Experiment playing both across the neck and linearly (in direction of the neck)
Learn, and experiment with, double stops (3rds, 5ths, & 6ths).
Ben has a nice pick exercise for the mando too that I found ot be useful.
hope this gives you some ideas…