What's best to add weight to hollow plastic table legs?

Cat litter will also work well

Depends on how breezy it is by you. I would go with cat litter (the old fashioned clay type), pea gravel or sand.

Still buy an umbrella stand too, it makes a world of difference. They have models from very inexpensive (that you fill with water or sand) up to very expensive styles that are quite pricey.

You can fill a mason jar with small rocks, then pea gravel, then sand. Weigh the jar with each of the items and see which one weighs more.

I wold use the sand as it would be easier to put into the legs and would fill every nook and cranny.

Well, I would choose pea gravel because small rocks may not be heavy enough if they create air space between pieces. Moisture in sand won't be able to dry out if any moisture can get in...even humidity can create mold. Curious about your tables...have never seen ones that could be filled for weight!

Sand. No air space like stone or pea gravel

Mold feeds on organic materials Clean sand is inorganic. A bead of silicone caulk before bolting the legs on would create a seal from water intrusion.

Sand would be the heaviest, pea gravel less messy and should add sufficient weight.

Will be outside so will probably use combination. What's a little mess outside anyway. Mud, now that's a mess. 😁

I so very much appreciate everyone's help!!! Thank you!!!

Sand or kitty litter would work. Use a funnel to fill.