All 7th chords / all inversions round the cycle — 20m
Harmonising the major scale with 7th chords — root inversions (20m)
Walking bass lines (Scott Devine) — 30m
Improvised bass line over jazz standard (The Preacher by Horace Silver)
Soloing over (slightly modified) 12-bar blues progression trying enclosures and chromatic passing tones
RealLifeReading™ — LOL — just picking up a random sheet with a bass transcription for a popular / jazz song and trying to fluently play and / or learn as much as possible within 30 minutes’ time. I think it’s a bit closer to the real life sight reading situations than endlessly going over contrabass etudes. Hence the name