Anything to improve? Maybe the background? One of my first renders
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Hello Simon, thanks for sharing your work, this is a nice beginner picture, you got there. 
I like that you have given the half sphere a bit of bevel on the edges, so it is not too sharp. The texture on the weight looks a little bit stretched, If you are using an actual image texture, I would try to scale it in the UV-Editor to get the size of the texture more even.
The knot of the balloon is too small, in my opinion.
The clouds look very promising, though also too dark, to be the type of clouds you are presenting them as. I would go with a bit more blue background and the clouds being more white.
I like that you sort of kept to the rule of third, but the question remains, what is it, that you want to say with this picture? There is not really one only point to focus on, you have the balloon and the handle, if you want the focus on the balloon, give it a color that stands out, complementary to the blue a yellow, or a red color. If you want the focus on the handle, do not put anything distracting next to it, like a balloon. Better put it in the background and let it swift away with some camera blur.
Now some general words:
I would at some point always try to connect your practice to a real project. For example, you want to practice to make this weight, then go for a realistic scene, a gym maybe, then a closeup on multiple weights stored in front of a mirror, or something. Then, you don’t only learn how to create a weight, but also something about lighting and modelling other things, and also rendering. Using the right perspective and focus, because real life cameras can only focus on one thing, so normally, the balloon in the background would be out of focus.
I hope I could help a bit and I wish you a merry christmas time and a lot of fun with Blender! 
If you want to stick with the “Lightweight” theme, how about making the weights balloons?