How old is she?
If she's 5 or older, then lunging is great. But a young one doesn't need stress on his/her joints.
You could also ground drive, or lead in hand and do some obstacle courses or set up trotting poles.
ON the horse, you can of course ride w/t/c and work on transitions, some leg yielding, a few half circles, a few 20 m circles, some western patterns, serpentines, whatever you feel like doing.
Always exercise to a sweat if you're working on building muscle. Just cool down and warm up nice and slow.
"Strength is the ability to use a muscle without tension"