take the harddrive out and either drill a hole through it or chop it up somehow prior to disposing of that separately. any drive formatted and reinstalled can still have data taken off it if someone is so inclined. donate whatever else you want but not the drive.
Nobody is going to bother to get back data from a zero/random filled drive on the offchance of finding something. Simply not worth the effort + huge expense.
Seeing as OP has Ubuntu already it is just a simple case of booting from the CD, and doing something like dd if=/dev/random of=/dev/hda.
That way the people you donate it to don’t have to fork out for a new HD as well.