Do you know the ins and outs of search engines and have skills in platforms like Google Analytics? The owners of a lot of smaller companies don't realize how much of an impact search engine optimization (SEO) can have on their business. Educate those business owners on the power of SEO to help transform their websites into a more SEO-friendly property. Use your skills to show business owners how to read and use their analytics data the right way, and how to properly use keywords and structure content to get more traffic.
Set up a merchant account. Service businesses in the past had to generally rely on cash or check—setting up an entire credit card processing system was a thankless, expensive task at best. Using a service such as PayPal makes it possible to accept virtually any form of credit or debit card for your services, and includes dispute resolution should the need arise (and it will arise).
I surprisingly get a lot of people asking to detail their car. I never intend to make a business of it, but I love doing it to my cars and people ask me to do it to theirs. All it takes is a cheap orbital buffer (mines a used craftsman) and a shop vac. I normally get easily $100 for a basic wash/wax/vac, or $200 to remove scratches and polish then wax the car.