Personally, I push a sled (Prowler) for GPP, do 50-70lb weight vest walks, or walk/stretch/locomotive movements. The lower arms are fairly small muscles so I try to only train them directly 2x per week, following my standard fitness. I'm not a grip master though, maybe @Joseph Sullivan or @Jedd Johnson can drop some real knowledge.
I'm more of the opinion that rest days are meant for rest, stretching, or locomotive activity. If you never rest, your body can't get strong. Without knowing the rest of your workout routine and general strength, it is hard to define. If you are training grip 2x+ per week and wanting to do farmers, I would advise against it. Then again, I'm a grip noob. What do I know?
Today's training (25.4.2019.):
Good morning: 90 kg x 3; 110 kg x 1; 125 kg x 1; 135 kg x 3; 145 kg x 3; 160 kg x 3 all time PR; 90 kg x 5 very deep
Wide stance box squat: 110 kg x 3 x 5
2h pinch: 62.5 kg x 5 x 5 PR
Very, very hard workout. The good morning was more of a squat/gm hybrid, but still hard as hell, especially beltless. My grip is advancing extremely fast I pray that my good health and lack of injuries lasts for a long time.