Street performer - Accept tips and sell things while your at it such as your latest CD, Sheet Music, How To Play Piano DVD or hand them a brochure with a pitch and links to an online site.
Piano Teacher - Easy to imagine. But you can specialize to tap specific niches to greatly lessen number of competing teachers.
YouTube Channel - Get paid per view for playing songs, teaching, reviewing music, reviewing music products. I wouldn't recommend using the Ad route because it interrupts your videos. Include Amazon Affiliate links; Patreon; Paypal; Sheet Music links; Link to your website, etc in your video descriptions for additional monetization.
Sell your CDs, DVDs, MP3s, Vinyl (records), Cassettes (yes, they're back in now), Sheetmusic on Apple iTunes, Amazon, and other shopping sites.
Get paid for Streaming Music.
Perform live in clubs either as a solo artist or with a band. Cover music and/or originals.
Create music software for learning piano, composing, etc...
Create your website to Sell Merch - T-shirts, Buttons, Stickers, Fridge Magnets, etc... Or sell from Amazon, Etsy, etc...
Use Crowdfunding to bankroll your musician ideas. Same can be done with Music Grants and Sponsors.
Create a monthly subscription fee for access to your best stuff. This works best once you're fairly well established.
Licensing and royalties when others use your music on their sites, channels or in other ways.
I've done several of these things in the past. They all work. You just have to study and develop some skill sets and then plan how you will do it. It's fun and the income is good.
Please add to the list if you have other ideas. We could all use some musicianship income.