Private tutoring can be a good source of income -- getting pupils is difficult at first, but most of them will, once started, have many lessons, so there is some regularity in your income. If you want to reduce your hours, raising your rates can do the trick nicely (since you are an expert professional, you can and should charge a high hourly rate in any case, otherwise people will assume you are not very good).
Proof-reading and editorial work is flexible (because you can do it at any time of day or night), although it does not tend to be particularly lucrative. That said, if relevant to your field, a track record of such work can do wonders for your reputation and visibility.
Other specialist freelance activities relevant to your field. These tend to be brief and one-off, but can be very enjoyable and quite lucrative.
[NB: since all of the above tend to involve working in a self-employed capacity, it is your responsibility to register with the relevant tax authorities, file tax returns, and pay the taxes (this is one reason why self-employed hourly rates tend to be significantly higher than employed hourly rates). If you feel uncomfortable doing these administrative tasks yourself, you may want to hire an accountant.]