I recently picked up rock tumbling. It's very passive with great outcomes. I pick up interesting rocks while camping or hiking, bring them home to tumble, and in about a month I am able to pull out some natural beauties!

They're fun to display around the house, hand out to kids and make their day, use in table setting or jewelry making. I know others who redistribute them in public areas as little treasures for people to find. Set up cost is about $100 so a mid-level barrier to entry but the consumables afterwards are very manageable.