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.)