The best alternative to tests is hoping that you're not going to be working on this anymore in 5 years, I guess.
- Code reviews.
- Maybe a linter can catch some of the easier bugs.
- Good use of type system may be even better than tests, but takes quite some practise and time, so maybe you can't afford that either.
- Testing manually is cheaper short-term - it's usually faster to test manually once than it is to write a test. The problem is that testing the thing again for every release isn't.