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The union thing. That varies state to state. It used to be, for example, that you needed to be a union member to teach in Michigan. This may no longer be true. But Florida is a right to work state. State law forbids any employer requiring union membership to obtain any job there. So there is no requirement to join the union, and little pressure to do so. (That level of pressure may vary by county. Many people view the teachers' unions in Florida as rather ineffective and unless you feel obligated to do it out of historical reasons (eg. Dad was a union pipefitter, and so I should join the union too...)).
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As Terry wrote, private schools don't have a requirement for a teaching certificate. Certificates are for K-12 public schools. (Private schools would prefer certificated teachers, but if you have an available expert in a field and s/he isn't interested in running through the hoops and putting up with the BS of obtaining the certificate...)
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If you have a Masters (or higher) in a subject (eg. Math, Physics, Chemistry, not in teaching) you can get a job as adjunct faculty / assistant prof in a community college.
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Gifted or after school or during summer "enrichment" programs. There are companies which do this. There is one I had some interaction with a few years ago. The name escapes me. If I recall I will edit this.
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Many schools are desperately short of volunteers. But be aware that in many states you may have to pay $30-75 for an FBI fingerprint / background check before they will let you on school property without an escort. (Some districts may reimburse you, many won't.) You don't need to be completely free of any criminal offenses (eg. traffic tickets are not a worry), they must be (1) non violent (2) not involve children (3) non sexual. If you got drunk at the AC/DC concert freshman year and peed in the bushes and were ticketed for it by the cop who saw you, you may be a sex offender ("indecent exposure", even if you are not required to register as one) in some states.
As for Unconventional Ways: One friend recently retired from engineering, moved to a new community, struck up a conversation with a neighbor. A few weeks later the neighbor, a school principal, asked him to come teach (he was unable to get an education major math teacher to even come interview at his small town school). An even closer friend, underemployed bagging groceries at the time due to layoffs from an IT company, was dating a newly minted teacher who had sent resumes out shotgunning and had just accepted a job offer that morning. They went out to lunch to celebrate her new career. Over appetizers a principal called the woman to ask her to come in for an interview, she said she'd just accepted a position elsewhere, "but here, talk to this guy, he's interested in teaching" and she handed her phone to the guy (my friend) in the middle of a restaurant. Two days later he had an interview and was hired on the spot.