Whats a boring business that makes money?

14 year business owner here. I sold my personal training studio and it was a great chapter of life. Ive been able to take home 200k+ most years and never under 100k after the first 3. So it was decent.

However, I fell more in love with business than I did fitness. Also I'm burned out on selling something that most human are so fickle and resistant to (eating healthy and working out). I'm burned out on being in an industry that relies on so much on paid advertising and gimmicky marketing.

Business in general is exciting to me. The business of things is more exciting to me than the actual product or service. I looking to hear from those of experience on some straight forwards business's that can succeed on hard work and elbow grease.