Since someone already mentioned dashcams and cast iron, my vote is: a bidet toilet seat attachment.
Fancy ones can roll $200+ but a super simple one with just a cold water hookup and no electricity will knock you back $50-60. I bought one right as covid was beginning to hit the west coast, and instantly realized I could never - ever - go back.
Huge bonus of the toilet seat style bidets is that, since you aren't actually replacing the toilet, they take like 5 mins to install and can be done in a home, condo, apartment, wherever.
100%! It's wild to me that the majority of Americans are just OK with not using one. Reddit introduced me to the concept and I have never looked back. You can get a super cheap introductory one on Amazon, but I would personally recommend the one by the Squatty Potty company, it's superb quality.
While you're on the website for Squatty Potty, their namesake is another must have under $100. It revolutionized how I use the bathroom, and I have one in every bathroom in my house. If I use the bathroom at work, I end up sitting there missing my stool and bidet. It's hard to go back once you've gotten closer towards perfection
Please post a tiny instruction sheet in your bathroom for confused guests. They can be confusing the first time with no guidance. Also it's very confusing that normally you don't push the bidet button.
I favor the Tooshie Classic. I had some bad luck with a previous cheap bidet. Either I was getting a tepid fairy spray or a firehose enema. The Tooshie ramps up at just the right amount to give good control. Spend a few more dollars and you get the Tooshie Spa with the possibility (not obligation) of a hot water hookup.