Seems like almost every guy at the age of 16 has acquired a set of adjustable dumbbells. If you still have them, remove all the plates from one side and put just the smaller plates on the other. This makes a nice tool for wrist exercises.

Rotate between supination (palms up) and pronation (palms down). This is a real burner on the forearms, and again, don't neglect your radial and ulnar deviation.

Light weight and high reps is the best way to go with these exercises. Three sets of fifteen each arm is great. Even if you don't really want to dedicate part of your session to it, you can just throw it in when resting between sets of squats. It's not that taxing so it won't be detrimental to your next set, unless it's one-rep max day.

Don't have changeable dumbbells? Grab a sledgehammer. You can get creative with it and increase the difficulty by moving your hand further down the handle. Just be careful of your face.

Related:  Stronger Wrists, Bigger Lifts

Related:  5 Grip & Forearm Exercises That Actually Work