Grow weed. You have a ready customer base with your students.
I suspect you are joking, but just in case you are not I would advise against this in the current situation in Colorado - the groweries and dispensaries there are producing very high yields of very high quality marijuana, and it is not likely that somebody new to the industry would be able to establish themselves into a competitive grow operation with zero prior knowledge, contacts or sources for good strains and genetics.
You have mentioned before (IIRC) that you enjoy a lot of hiking in your area. How well do you know it? Well enough to turn over a decent secondary income as a hiking and/or hunting guide? People pay a lot of money for this, if you are able to market yourself well.
Some people have suggested the security sectors, and corrections. These are also viable options as many people do enter into these career paths later in life - as a kind of "sea change". The Private Military Contracting sector has a lot of options if you wish to pursue this kind of a path, and it is not all crazy Blackwater stuff. Many companies offer entry level positions with no prior experience working offshore security on oil rigs, and/or merchant ships.
Private investigation is another option you may wish to consider. Nothing like old Humphrey Bogart films or Tom Selleck's mustache of course. But investigation of insurance claims, unfaithful spouses, cooperating with local LEOs in keeping tabs on certain offenders. Some friends of mine work in this sector and make a comfortable living off of it - although it does require some odd hours.
The best advice I can hope to offer you, is to find something you love to do. Something you are driven and have a passion for. Then find a way to market that to the public so that they will pay you to do it. Start off doing it as a secondary income generator with minimal investment, the fact you are doing something you enjoy means you will not mind using your recreational time in which to do it. Trial and error, and see if it floats or dies.
All the best in your endeavors.
