It would depend what ride, what time the park opens that day, the size of your party, and where the FP is relative to C Palace. Probably you'd do better though to allow that time as a buffer.
If MK opens at 9am, and you are at the MK front gate (through security) right at 9am, there's a few options that you could do at 9am, such that you'd get back to CP by 9:30am, but not many. You'd have to choose something close, short , little wait.
I'm sure you could hop on the teacups, for example, but you don't need a FP for those at 9am. Many FP e-ride queues take 10-15minutes; Space Mtn. can be even longer.
Another good options is to arrive at CP before 9:30am, like 9:15am. Depending on the size of your party, there is a good chance you'll be seated early.
Transportation and security timing are both somewhat unpredictable though. I'd tend to simply allow myself a buffer over a FP.
Upon entering MK, you can easily take some photos and kill 15minutes in the shops.
If Mk open at 8 or earlier, then yes, you could do a ride before 9:30am. If it is an early entry day, or EMH, then many lines will already be substantial by 9am.