Already good answers to this, so I'm putting out a list of things I've done to add to ideas for possible music income. Some have already been mentioned.
Tutoring - good for steady income, but sometimes hard to do if you have a performing schedule that takes you away for some weeks. Students stick with you longer if you have a regular schedule with no long breaks between lessons.
Selling Product - This is changing as music is distributed more on-line now, but producing recordings and promotional products can still generate income. I have music up on-line from a show I did almost twenty years ago that still produces a small amount of money each month. At physical shows people still like to buy souvenirs from the band, so shirts, stickers, videos and recordings all can generate income.
Wedding/party band - Standards or covers are usually requested for this. You can either work your own group, which can be promoted through various planning services and web sites, or hire yourself out as a sub-in to multiple groups (or both). Wedding music tends to be seasonal. Businesses often hire cover bands for corporate parties such as the Christmas party, product launches etc. For weddings I worked in a cover band as well as a "theme band", playing "Celtic" music, and subbed in with a Country band.
Theme Festivals - This depends on where you are, but in some places specific styles of music are popular. For example, I had a "Celtic" band that played at Celtic fairs and other theme events. In some places Jazz festivals are popular. Occasionally we did well setting up a vendor booth for the street fairs and selling our recordings out of the booth. Handing out promotional material also lead to private bookings for the band.
Session/studio musician - Also changing now as small and home recording becomes more popular. In the area I lived there were a number of recording studios in travel range for me and they would occasionally need musicians to come in for session recording for projects.
Theater musician - Some theater productions still use live musicians. Depending on Union status in your area, you can pick up gigs for a show. It tends to be much time for little money though, as rehearsals aren't paid or don't pay as much as show times.
Busking - Rehearse your material and get paid. If you are working Busking specifically for money you have to put on a show about it, pick the right material and play to your audience. Tourist areas are usually good locations for busking.