There are a lot of things you can do to be always learning and challenged that don't require too much effort or even equipment. There are various ways you can figure out how to get what you need to do them for relatively no or low cost if you need to. Sometimes it's as inexpensive as simply thinking, for instance questioning why certain things are the way they are. Some call this pondering.
Anyhow, here are some other ideas:
Read good books, fiction, non-fiction, educational, classics, history, so-called "light-reading" and "How To" books.
Learning how to use or improve your use of a computer, netbook, tablet computer or smart phone.
Making or creating something...the possibilities here are great but the end product doesn't have to be great. This is one time the very effort often is the worthwhile part, not the end product.
Mind Builders: Puzzles, Crosswords, Scrabble, Brain Teasers.
Learn to sew. An entire outfit if you want but my suggestion is a sewing on a button, fixing a hole, darning a sock, stitching up a tear or split seam.
Learn to use tools. There is nothing wrong with not knowing how to hammer a nail, saw a piece of wood, use a screwdriver, pliers or a car tire jack. There is everything right about learning how to use them.
Learn to cook. It doesn't have to be fancy. A burger, rice, pasta and even stir fry vegetables is not so hard.
Repotting a houseplant or planting a garden, no matter how small, figure out how to keep it growing.
Learn another language.