Both of the tripod legs I have had allow you to hang something off the bottom of the center column. The usual recommendation is that you use your camera bag to help stabilize it, especially on a windy day.
Personally I have several issues with this. My new Manfrotto tripod has some clever mechanism or something because for the life of me I CANNOT get the hook out of the bottom of the center pole. Most frustrating.
Second, when I take my camera and lens out of the bag, the bag is now 75% LIGHTER than it was, and not that much use as a stabilizing device.
Thirdly, because the bag is so much lighter, it tends to be caught by the wind and sway, which adds vibration to the tripod setup.
What to do?
Well I got a landscape photography magazine last week, a british one that did a landscape feature and it had Tips from the Pros, and there was one really clever one that partly solved all of my problems – instead of hanging the bag off the hook, leave the bag sitting on the ground and use a bungy cord down from the hook, to provide better stability!!!
Brilliant! Now I either need to start filling my bag with rocks to make it heavier, or buy MORE lenses 🙂 Just as ballast LOL!
Here is someone who had a similar idea
And some form of metal weight here – from a piece of gym equipment?
Learning to See Light 
Something else to consider for extra weight is water bottles. Somewhat more practical than just carrying rocks, because if you get thirsty you can drink them, or if they just get too heavy you can empty them out pretty much anywhere ( “Hey, what are you doing dumping rocks in our parking lot?!” ), and if you are someplace with water available, you can carry the bottles in empty, fill them for stability, then not have to carry the weight out.
Bottles of water is a good idea for all the suggestions you make. I usually have my car with me so carrying a rock around isnt a major issue, its the WALK from the car to wherever the tripod needs to be 🙂 I liked the three legged sling idea in one of the links, that seemed to be quite a sensible idea