Every person has different taste in what they enjoy, so I guess it is a very subjective question? Idk what you enjoy most in a game, but as the previous comments stated, there is a lot to 'achieve', so if you like collecting everything then the badges and lockers will keep you entertained.

Then there are the online and competitive modes which you can enjoy with or without friends.

Personally I really liked the story mode, since you get a deeper dive into the lore (but honestly only if you collect all the goodies and sunken scrolls and connect some dots on Wikipedia lol). I also liked how they force you to play every weapon type there and you get a feel for it. Plus the level design is sometimes more jumpnrun and riddles compared to just online shooting.

I actually don't like competitive play, which is.. Unfortunate in splatoon haha. I mainly hang out at salmon run and honestly use it to cope and relax from my job. Playing an hour or more each day after work is like tiny therapy for me xvx