When I was thinking for some not most obvious hobbies for my sims I came up with these ideas:
1. Simstagram - sims take pictures of pets, selfies and just misc photos and post them online, with variants such as travel bloggers, food bloggers etc.
2. Debating - I have a debate club in my game aside from the Uni one - this also includes research, browsing simpedia and so on
3. Dog training - I wish it works better for inactives too. My dog parks (with dog-welcome trait) with training courses are always empty and no one beside active sim interested in them
4. Bird watching (and collecting feathers)
5. Entomology - collecting insects from Outdoor Retreat, grub farms
6. Dancing (even if I had to pretend that doing modern moves is practicing ballet)
8. Karaoke and singing in general
9. Home keeping - these sims not only will clean but also record "organizing", "cleaning", "cooking" videos, practice decluttering (Eco Living recycling or donations or street sales), upgrade home appliances and so on.
10. Fashion - sims will have and use vanities frequently, shop for clothes, try on outfits, again doing selfies and will be involved in related careers most probably
11. Movies/obsessing over celebs - claim favorite movie genre, popcorn movie nights, always use discuss movies interactions in the process, have movie posters as deco, take autographs, TV premiers... all of that stuff
12. Cosplay - club "Cosplay" for that, Geekcon, JTBT stuff
13. Gambling - card tables, lottery and again JTBT sabacc (which basically is a card gambling and can be set on any card table)
Also since there's no proper hobby enthusiasm attribute in game (I hope yet) I group my sims in clubs by their hobby thematic. This allows me to remember and track it as well as push them to do more hobbies related stuff during gatherings.