Can I save activity Monitor data?

Take a screen shot of the Activity window by holding down the AppleShift3 keys.

You could use the top utility from Applications -> Utilities -> Terminal. It has a logging mode

top -l 1000 -ocpu -R -F -s 5 >top.samples.txt

Which will cause top to collect a sample every 5 seconds for 1000 samples, and store that information in the file top.samples.txt

You could write a script that collects ps output

#!/bin/sh
for (( j=0; j < 1000; j++))
do
 ps axl
 sleep 5
done >ps.samples.txt

Which will collect a ps sample every 5 seconds for 1000 samples.

Information about the top and ps command can be obtained when in a Terminal session via:

man top
man ps

This does work on 10.5 if it is what you are looking for.
http://www.versiontracker.com/dyn/moreinfo/macosx/21593
then you would have to do a simple division for %. You'll see what I mean.