I collect video games and have now for years. I used to collect CDs but with some exceptions, stopped, as I prefer to have them digitally for listening or on vinyl when available as I do not care for them much anymore. Yes, I know you can simply download and emulate (most) older games. However, unlike games, which actually get used and provide a more genuine and unique experience when played on the original hardware (the Virtual Boy for example) CDs do none of that and often just sit there, taking up space and never being used again after being ripped. Also, music CDs tend to, on a larger scale, have much lamer artwork than most games do, and I could go on about why I prefer collecting games instead.
Anyways, aside from games, I collect a variety of niche things including particular books and records. For books, it can be either fiction or non-fiction in some specific subject like hanafuda, reversi, calligraphy, foreign languages, programming, etc., and with LPs its stuff like 12" singles.