We got an 18'' planter and filled it with a 60lb bag of cement. We had left over 1 1/2'' PVC pipe from a plumbing project, which we inserted into the wet concrete.
After that set for 24 hours, we spray painted the PVC, added gravel on top of the cement and 4 little planters.
Much prettier than a weight, right? And it cost less than $20.