At the most, you can upgrade the RAM memory.
The iBook has built-in memory on the system board, which may be 128MB, 256MB, or 512MB
There's only a single memory slot for additional memory, which can take up to 1GB, one stick of PC-2700 SO-DIMM.
So, depending on which iBook G4 you have, you could go up to as much as 1.1 to 1.5GB maximum RAM.
And, you can replace the hard drive, with more capacity.
It takes a 2.5-inch, IDE interface drive - and replacing the hard drive is very challenging
(Count the screws - I have done that! - there's 50 tiny screws to remove to get the hard drive out - lots of fun.
Other than hard drive and RAM, there's not much "modding" you can do to an iBook.
And, you can upgrade the OS X version, maximum of OS X 10.5.8
That's a good reason to want to maximum RAM installed.