As an aside, I feel compelled to defend a profession you referenced as involving "little [...] intellectual effort."
Waiters and waitresses cannot be intellectually dim and still perform well. Good servers have both intellect and empathy. Sure the profession is generally looked down upon, but there is no time to daydream. Ask any server, it is not the tasks that are unappealing, it is the customer. The ungrateful, rude, entitled customer. Because you can earn
20−30/hr+ in some places (min wage +tips), you might find some really intelligent minds competing for these positions.
The grocery store counts as menial, but not comparable to restaurant server. A smart/creative mind will find ways to make any menial job more exciting. One could, for example, try to memorize the exact price of every item at the grocery store. Try to guess the total amount before scanning items.
It's rarely the menial-ness of the job that controls satisfaction, because all jobs eventually become menial. Novelty wears off all things inevitably. It is the compensation for doing the menial job that will make someone content or ecstatic to go to work tomorrow morning.