If you are careful, Bridges with your head on the edge of a bench/sofa can be good. Hold for 20 seconds.
Stable, isometric strength without the danger of traditional Wrestler Bridge.
Shrugs for upper trapezius. Best with arms out to side slightly.
Not holding your head when doing crunches, directional movements using your arms for resistance, etc.
Essentially, people don't usually use their neck muscles much at all so any real use will give results. But the most important thing is safety. Seven small bones, important nerves, eight small disks. If you mess anything up or overwork it can lead to osteophytes (bone spurs) down the road that can cause a world of hurt. So be careful!