Mainly drama CDs, original soundtracks, and official artbooks. I rarely buy small trinkets like acrylic stands or badges, as I prefer media that lets me revisit the voice acting, music, and detailed illustrations anytime.
I also collect a lot of booklets and CDs of titles I really love! I just can't get enough content from the main games and have this compulsory need to know everything there is to know about them (the beta works, artist/writer/seiyuu comments, etc.), also why I collect a lot of art/fan books. I collect little trinkets too only when the art is original and captures my attention, but I mostly like big ones like tapestries and cloths that I can hang up. Nuigurumis too, of course, it's cute displaying them in a corner.