Weird, I was just thinking about this very topic this morning. This is stuff you can toss in your gym bag.
For lifting: belt, chalk, wrist wraps if you have girly wrists, neoprene sleeves if you have bad elbows/knees
For recovery/prehab/rehab: small hard sphere, PVC pipe, band set (mini through strong)
For lifting: IronMind Sew Easy straps, knee sleeves, knee wraps, ammonia, dip/chin belt, front/zercher squat harness, weightlifting shoes
For recovery: The Stick, rumble roller looks awesome, massage
Specialty bars, not for your gym bag
Yoke bar or SSB, Giant cambered bar, deadlift bar, and chains for chain suspended work.