Measuring low ripple on a power supply with an oscilloscope
I have a similar scope. Not a bad little unit but I have to take the front panel off and clean all the buttons. :-)

You've already figured out that accumulate mode will give you a good idea of the "envelope" of the ripple over time. In order to get a clearer picture than that you'll need either a better scope or amplified probes, but I'm not sure why you'd need to worry so much about that ripple. Is this merely academic or are you trying to do something unusual?