SpongeBob and Patrick should just have their story arcs told with in the main series. Kamp Koral could have easily just been a mini series with in the main series without aging down the characters and no CGI. Yes, summer camps for adults do exist. Spin-offs should just be used for supporting characters.
If SpongeBob had a really strong secondary character (or two), this might actually be a great idea. Happy Days had a few really successful spin-offs because they had a couple of really popular (and funny) secondary characters.Make a show about Fred or Bubble Bass. Old Man Jenkins too.
Ever hear of Robin Williams? He made his big break as Mork, a one-off character on Happy Days that proved so popular they gave him his own show, Mork & Mindy.
Unfortunately, I don't know if any secondary character has the mass-to-volume ratio to attract that kind of success. My best bet would be Sandy (although I still consider her a main character). Perhaps "Sandy the Science Squirrel" or "Sandy's Ranch"... that sort of thing.