[...] the actuation distance need not be great, one cm or two... but it would need to be fast. The weight of the robot would be a few kg at most.
The hopping amplitude would be built up over time to a harmonic peak.
You already have most of the solution then - harmonics is the way to go, to keep the average power requirements bearable.
Not having seen what you plan, my immediate thought is to use a large (home made) solenoid, with a (magnetically shielded) coil in the main body, with the force being transfered through the leg via a (carbon fiber?) tube to a reasonably hardness synthetic "rubber" "shoe" ( a brake pad fom a side-by-side roller skate sounds about right).
The outer shell of the leg doesn't need to be super light, but the actuating rod should be as light as conveniently possible and don't make the leg too short, that will make it less "stable" - think of it like balancing a hammer on one finger; with the head up it's easy, the other way it's "regulation time" becomes too short.
You should experiment with the stroke length vs. supply power - longer strokes can be relatively slower for the same result (jump-vise).
Trigger timing will take either optic- or Hall switches to determine the position of the "kicker" (both for the trigger signal and to determine when a full stroke has been reached). Done that way, it will automatically adjust to the harmonic frequency of your jumper, even if you add or subtract some weight or a tether (and you'd need an accelerometer to determine which way to angle the leg of course and the leg needs a 360° moveable hinged connection to the body.
Add plenty of capacity to the solenoid supply, to make it kick fast and powerful. The coil will depend on your voltage and power switch element.
For short run times, a small CO2 bottle could be used for pneumatic actuation, but pneumatics valves and such are costly.