I remember seeing old photos of my great uncle's carnival when I was little. He was a stage hypnotist back in the 1930s. One of the clearest pictures in my mind is that of the strong man and his incredibly heavy dumbbells. These are theatrical dumbbells made to look like the type used in circus theatrics at the turn of the century. They're easy to make (if not a bit time consuming) and a lot of fun to age paint.
Step 1: Paper Mache and Finishing
Unfortunately I don't have many photos of the process because of all the glue on my hands and lack of friends. However! This is pretty straight DIY fabrication. If you want absolutely round weights, you can use 'gender reveal' balloons as they are heavy latex and intentionally perfectly round. They're also very unlikely to deflate. You could also use an inexpensive bouncy ball from a department store like target. The ball that kids always want to knock over a vase at home with (sorry Mom)
I used newspaper strips and elmers glue with water added. This varies by climate but I found 2/3 glue to 1/3 water worked well for producing a shell with few layers. Remember to criss cross each layer for strength and use tear every side of the strip you're using. Straight cut edges stick out.
For smoothing I used wood filler. After applying a layer of wood filler, sand it down, spray paint some grey primer and let that dry. Now examine for any areas that may need filling. Repeat as needed.
I used plasti-dip as it gave a wonderful dull heavy look. Once it dries, you can sponge on silver paint and wipe it away. Leave it in cracks and imperfections. Next, you'll want to mix some orange and brown acrylic paint to create a rust color. Save some orange for the rustier bits. Less is more with rust and it should be placed strategically. You can also mix sawdust with elmers glue and ugly up the texture in areas you want more age. Stencils for letter/number of weight are fun. Aged stencils are available at craft stores.
I used expanding foam inside the dumbbells to prevent collapse if they're dropped. I do recommend this but be careful. This stuff is very messy. I used PVC pipe and there are a million ways to do this. I do recommend running the pipe through the paper mache housing completely and having as many anchor points as possible. The foam seals it in place nicely. Mine is modular and unscrews but that is not necessary usually.