From a similar post recently:
Inexpensive, and little setup, with no steep learning curve?
Off the top of my head:
- singing
- drawing
- playing a tin whistle
- dorodango
- dexterity games (card flourishes, astrojax, yoyo, pen spinning, etc.)
- reading
- kumihimo
- customising things with paint markers
- yoga
- whetstoning
- writing (prose or poetry)
- journaling/journalism
- cross stitch/embroidery
- jelqing
- rock/object balancing
- meditation
- knife throwing
- macramé (using beading floss)
- playing the spoons
- whittling
- eating
- scrimshaw
- nålbinding
- henna tattooing
- primping
- cloudgazing
- looking at fucking everything through a high mag loupe
- tarot divination
- contact juggling
- kefir making
- key duplication by hand
- sleeping
- hiking
- floccinaucinihilipilification
- origami
- talking to nearby friendly looking strangers
- polishing
- cleaning
- caligraphy
- drugless hallucination
- soapbox public speaking
- zen gardening
- guerilla gardening
- pitch ear training
- 'easter egg' hiding
- local botanical/ecological studies
- flint knapping
- home/interior design (as long as you don't actually buy anything)
- knolling
- marble running
- solo RPGs like 'Alone Among The Stars' (okay this one wasn't technically off the top of my head. I couldn't remember the name, so I looked it up.)