If I had to do it all over, I would design medical/surgical equipment. My first job after my ME degree was to get into the design of UHF and VHF TV transmitting antennae and FM (circular polarized type) antennae. It was a great job, however I busted my leg while ski jumping and I had to find a different employer.
In the world of mechanical design, the work is challenging whether it is in the design department, manufacturing department or getting involved with the purchasing department that is always trying to sabotage your designs with cheaper material.