I don't know who decided way back in the day that crunches were a good idea, but they're more or less useless. The range of motion during a crunch is incredibly limited, and the exercise fails to engage the deep, stabilizing muscles of the core or back, preventing it from offering a functional advantage. Skip the crunches and opt for full sit-ups or better yet, planks -- each will provide a functional advantage and full engagement of the core.
There's no reason for the average Joe to do forearm curls for basic physical fitness. While incredibly important, the small muscles of your forearm are used during practically every upper-body exercise as you lift, press, and grip weights and other equipment. If you're concerned you're not working them hard enough, ditch the curls and opt for jumping rope with a weighted jump rope, instead. In addition to taxing your forearms, you’ll benefit from a tough burst of cardio, too.
These are the muscles that run along your shins, folks. If you're wasting time at the gym working your shins (the muscle running along it is called the tibialis anterior, for what it's worth), you better be trying to recover from shin splints. Otherwise, it's not a thing.
Like the muscles of the forearm, no one's saying the tibialis anterior isn't important -- of course it is -- but as a small muscle group that's engaged during all lower-body movement, you're better off putting it to use when you run, jump rope, play a sport, or ride a bike.