how can I wipe the data from an old mac?

Use the following instructions if they will start and run: What to do before you sell, give away, or trade in your Mac - Apple Support

If they won't, the only other option that I know of would be to remove the hard drive(s) or SSD(s) -- and the degree of difficulty depends on the model.

Hello,

For the Macintosh SE, begin by manually moving all personal files (including preferences), and everything else except a basic system folder, to the Trash. Empty the Trash (the Special menu).

A normal Macintosh SE would use 2DD/DSDD diskettes in a special 800K Mac format. The FDHD variant can handle 1.44 MB as well. If you have access to a Disk Tools floppy, run the HD SC Setup utility in order to erase the internal SCSI hard drive.

For extra security, a special wiping utility such as Norton's Wipe Info (commercial) or Burn 2.5 (freeware) may be used (providing that an appropriate operating system version is active). Alternatively, one can fill the Macintosh SE's hard disk with multiple duplicates of an unimportant large file (in order to overwrite free space), and thereafter again move the contents to the Trash (and empty the Trash).

Macintosh SE: Technical Specifications

Very helpful. Thank you!

If you have system disks for any of those, you can wipe the disk using those. However, all too many computers get separated from their disks during their service life.

The best way is to physically remove the hard drive. As no one will likely be able to reuse them due to their age and lack of software, that is a viable and 100% effective solution. Three are easier than the others.

Of the PowerBook, the iMac 27 (A1312 is a 2011 model), and the "aluminum G5" (which i take to be a PowerMac G5 that looks like this):

the PowerMac G5 is the easiest for physical drive removal.

PowerMac G5 remove drive

The 2011 27" iMacs have accessibles drive but require suction cups to remove the glass cover and the removing of LOTS of screws to release the display assembly for drive access:

27-inch 201l iMac remove drive

A Powerbook is a fiddly thing for internal work but doabe. Again, as this is a near zero chance of that computer seeing further service, it doesn't hurt if you break something:

PowerBook 15 remove drive

The 2000 iMac has me hesitant to recommend any disassembly attempt without considering both your skill and comfort levels dealings with computer innards that retain dangerously high-voltage due to the built in CRT display.

Likewise I have no experience with getting inside an SE, also with a built-in CRT and similar high voltage issues

For those, I would first consider a university's computer labs; some take in outside work for building student experience. The other possibility is help from an Apple User's Group:

https://appleusergroupresources.com/find-a-group/

Someone may be wiling to do the work free if you donate the leftover part to their collection.

Methods may vary depending on how old a Mac is involved. This forum section is for computers made before 1998. Are yours that old? If not, please post what specific models you need to recycle.