Selling Service Seasonally
Another option is start a seasonal business selling services, which can be operated with a full or part-time effort. But most are run full time to maximize revenues and profits over a normally short time span.
Examples of seasonal businesses selling services would include snow removal in winter, yard maintenance in summer in northern climates, income tax preparation service in spring.
And serving as a vacation property rental agent. Just about any business can be run seasonally or occasionally, but some are obviously better suited than others.