In the Great Depression, not many jobs at all. Remember that Lennie and George go to the Murray and Ready's employment agency and got work slips for the ranch. These were companies that you gave money to and they located a job for you, or employers requested workers and paid the company to find them.
In the Salinas valley of California, the work would be overwhelmingly agricultural jobs, working seasonal harvests (notice Lennie and George are migrant farm workers), orchards, strawberries, barley, etc. These jobs in those days were intensely physical, a lot of picking, tending, digging, cutting, harvesting, carrying. Tough work for $50 a month.
In Salinas Valley in the 1930s, most of the jobs were based on ranches. Lennie and George in Of Mice and Men work as itinerant ranch hands, and some people (very few in number), such as Curley, own the ranches. Other people are employed at different tasks on the ranch, including as a blacksmith, who creates objects from iron. Other characters in Of Mice and Men are employed as "swampers," (such as Candy) who do odd jobs such as mopping out the bunks where the men sleep. Crooks works as a stable hand, taking care of the horses, and Slim, the person everyone respects, is a jerkline skinner. That means he is the head worker, and he drives the mules on the ranch. There are also people who cater to the men when they go into town to blow off steam; these people might include people who own or work at restaurants and bars.
In the Salinas Valley most jobs that are available in the book are ones related to tending a ranch. When George and Lennie get to the ranch they work as field hands which means they would roll and stack (barley) grain or products. Lennie is very strong so he would be used to lift products onto truck beds. There is a ranch foreman who oversees the ranch hands tasks, a mucker, who keeps the bunkhouse clean, cook, who prepares meals for the men, stable hand, and driving the trucks. Most of the jobs are general towards running a barley ranch.