Get a piece of pipe or wooden dowel, maybe 1.5' in length, maybe 1.5" to 2" around. Take a strap(old ratchet strap or something), tie it to the middle of the rod. The other end, loop it over a 2.5lb plate(or a 5lb). Hold the pipe with both hands(strap in the center). Straighten your arms to shoulder level, start slowly rolling the pipe, which picks the weight up off the floor(strap needs to be long enough for the weight to be on the ground when your arms are extended). Roll the strap/weight all the way up till the weight touches the pipe, then slowly unroll it. Keep doing it. It will burn your forearms up and improve your grip(much of your grip strength comes from forearms). This also keeps your shoulders working and your core engaged as well.
Other ways to improve grip strength. Dead hangs from a pullup bar. See how long you can hold without dropping. If you can hold for :30 secs, go for a minute etc. Add weight in the form of a belt with a weight attached. If you do deadlifts, try and use a double overhand grip rather than a switch grip for your heavier lifts. I used to switch grip a lot for anything over 225. A PT friend of mine told me that you run the risk of popping a bicep on switch grip because it engages the bicep more. So I switched to double overhand. I can now comfortably pull all but my max weight on deadlift with a double overhand grip and that translates to a better snatch and clean and jerk(same hand position).
Forearm/grip strength is fun to train but can make humble you real quick. A 2.5lb or 5lb weight can get quite heavy