Tire balancing weights as others have said.

I have also used sand by pausing the print then filling up some of the voids left by infill. But you have to be really careful because you don’t want it in you belts or motors. I usually put the sand in a reusable condiment squeeze bottle from the dollar store and also use a small funnel to make extra sure its going where I want. I don’t fill all the way to the top and make sure all the fans around the printer are off while filling so they don’ blow it around… use tire weights if you can.

I’ve been a fan of making false bottom pieces and using steel wheel weights in them.

Here’s a lamp I made as an example.

If you’re using cheap filament, also consider cranking up the infill percentage. I recently printed something for a friend that is almost a 15cm diameter sphere at something like 85% infill. Every time I hold it I’m surprised by how heavy it is. I haven’t tried it, but I think many slicers will let you change the infill percentage after reaching a particular layer, so you can make something bottom-heavy to prevent it from tipping over.

Leave cavities for steel things like big bolts, shaped so the bolts won’t rattle around. Add a pause to the print before covering the cavities. Disable support for the cavities.

When the printer stops, stuff the cavities with bolts, then resume the print.

I don’t know a general approach, but in one particular print with low infill I paused the print to pour in some sand just to make it more bottom heavy. While it worked great in that case, I don’t know how well it’d work in other circumstances. Great cost/weight ratio, though.

I used wheel balancing weights (tiny 5g iron bars) and built them into my prints.

They are pretty cheap and the included adhesive makes it so they don’t rattle around inside if the tolerances of the cavity are not perfect.

Used them in the base of a tall slim model to make it less prone to tipping over.