Refilling my pen. I have a thing for pens. I always use the same one.
By the way, using a fountain pen is cheaper than you’d imagine. Of course it’s more expensive than a plain old Biro, but I have weird priorities. This is a cheapish Lamy pen and once you buy a converter you’re not spending money on cartridges either. Before I used a fountain pen (I am very particular) I always bought a specific sort of Paper-Mate pen anyway, which might only last a week. Also, less plastic waste! Last year I had an aluminium can leak in my bag and my notebook was completely written off, yet the ink did not run! Worth every cent IMO.