Upgrading barely-satisfactory household goods to better versions. Many such goods are bequeathed or obtained when the user can't afford better, and never replaced once they're in a position to do so.

Example #1: laundry apparatus. When I was younger and poorer I bought the cheapest laundry basket and airer I could get. They weren't very good, but I laboured with them for over a decade because they were satisfactory. A replacement set in my 30s cost me less than I would even notice spending, and vastly improved my laundry workflow and throughput.

Example #2: kitchen knives. It's alarming to me how many people think a bread knife and one other miscellaneous sharp knife constitutes a fully-equipped kitchen. If you spend any appreciable amount of time preparing food, and you only own one straight-edged kitchen knife you don't know the name of, you're almost certainly making life harder for yourself. Buy an inexpensive 5-piece block set and experiment with each type of knife on different foodstuffs.