...to fulfill their desires.
Personally, I like to do stuff that gets me out of the lab, not that keeps me in it for longer.
I'm a qualified archery instructor, and am just getting back into wildlife photography after a lengthy break. For a number of years I spent my free time touring by motorcycle, and carrying a heavy camera wasn't really practical. I'm also considering learning watchmaking later this year when it inevitably turns dark and cold outside.
Whether or not your contract is legally enforceable (ask a lawyer this, not a forum!!) may not be all that important. If you take on work which causes your relationship with your employer to turn sour, that in itself is a major problem that will keep you awake at night. It's not worth it, IMHO.
When I was last working as a full time employee, and was offered some part time contract work, I was totally upfront about it. I negotiated a shorter working week and insisted on a contract which permitted me to work elsewhere provided it wasn't in direct competition with my employer's business.
I also made sure it was clear that nobody had any claim over the IP that I generate, which is a hugely important issue that engineers don't always think about enough. Make very sure that nobody claims, or thinks they can claim, IP that you produce when working for yourself or for other people.