Memorization mnemonics. Shuffling through index cards to associate numbers to words. See Mind Performance Hacks (2006) from O'Reilly.
Make notebooks with cheap filler paper, high-capacity stapler, and duct tape. Use G2 pens or archival quality ink.
Make your own index cards: fold in half and then into thirds; cut those into rectangles. (Still on the lookout for good storage.)
Shade paper with crayon. Carpet is smoother than desks. Envelopes are a good way to iterate color combinations. Pick colors at random.
Convert mailed coupons into CSS. You get salable graphic designs and color palette for free.
Break down milk cartons for free cardstock.
Prime cardboard with paint for a cheap canvas.
You can start cooking relatively cheaply. I do remember having sharp knifes and a good cutting board, some decent pans and you'll have a good time.
I'm documenting graffiti a lot, but also taking pictures of other urban/nature/landscape scenes.
Took ten thousands of photos in the past years with a full-frame camera to the point that wielding the camera is second nature.
Also learnt editing raw files with a professional tool and trying to improve on it.
I hate taking photos with my smartphone now.
Humming to find the resonant frequencies of the space you're in.
Observational drawing- paper, pencil, draw what you see (or if not sighted, maybe there's a similar activity?)
Listening to what's happening around you. Originally thinking birdsong, but the lowest barrier is just where you are.