There are actually people who still want these newer PowerPC-based Macs. Some people might call the last G4/G5 Macs slow today, but with minor upgrades and optimization of the OS, these PowerBooks can be pretty speedy. You can even use Safari extensions if you have the latest version of Safari on 10.5.8 and completely nullify flash on YouTube and play videos pretty decently at 360-480p.
If you don't have anything to do with it, I would list it on eBay. I don't know what the screen size of yours is, but the 12" models are very desirable for whatever odd reason. I just paid $71 for 12" PowerBook G4 early 2005 in good shape, and that is cheap! They usually go for around $120 working.
The 15" and 17" models go for less, especially the very first or very last revisions of them, as the first ones are fairly slow and the last ones had that problem with the sleep sensor.
If you want to keep it, I would find a way to install an SSD. SSDs really pep up these older machines along with maxing out the RAM.