During the summer you could post fliers or go around and ask people in the neighborhood if they need their lawn mowed.
I earn $20.00 per week mowing each lawn 4 times per week and do 5 lawns per month so that's an extra $500.00 per month for part time mowing work.
A lot of people are glad to pay someone else to do their lawn mowing and some people may not be physically able to mow the lawn.
Other things you could do are things online such as taking surveys, doing freelance work such as making logos, writing blog content, updating peoples blogs etc.
You could also create your own blog and blog about a topic you know a lot about and overtime you can earn a good side income and even could potentially earn a full time living blogging.
In the beginning though you'll need to work hard long hours writing lots of great content and publishing it on your blog and then over the years you won't have to work as hard and the money can still keep coming from advertisements or affiliate sales etc.
This can be a good passive income but it will take time to get there but after all those years of hard work you'll reap the benefits for years and years to come as long as your blog is still online and continues to receive traffic.
To keep your blog ranking high you need to constantly add fresh content at least once per week.
You can also design and sell T Shirts online to make a good passive income from T Shirts that you design and let other print and drop ship the T Shirts to your customers.