I think most of the things I have are already listed, but something I've liked doing is having a tablecloth/scarf thing to match the character (in this case, a scarf with the pattern of Spark's dress and necklace). Also, if you get a clear photo frame from the dollar store, you can put a drawing of them in it (like the one standing up in the back) as a somewhat larger piece!! Besides that, the usual standard items (buttons, acrylic stands, keychains, etc) are a great way to go. Really, most of my collection is acrylic stands and keychains in various sizes and styles, but eventually I want to branch out a bit also! Would love to learn 3D modelling to 3D print a figure eventually.
I haven't done one for Spark yet despite having the embroidery floss in my sewing box, but embroidery patches is also a fun one. I did it for my sona a few years ago, and I still have it on my wall. Custom pop sockets are also a fun one! God this thread is going to fuel my custom oc merch addiction so bad aaahahah