Lots of the fireroads are mostly up (and then mostly down). From West LA, you could do Sullivan Ridge or (shorter) Westridge, but they are (mostly) up one way. Dirt Mulholland is a good suggestion, it's long (but I find it too boring). Probably the best for up and down is around the Hub; you can get there simply from the top of Reseda, go on dirt Mulholland for a little bit, then turn off to the road taking you to the Hub, and then ride around there.
For the Cheeseboro area, you could try Baleen Wall/Cheeseboro Ridge trail - a bit of up and down along there, and really nice views in places. Or go up Powerlines and all the way around to Alberson Motorway - many miles of fire road there, and drops you back in a really nice area.
Out in Sycamore Canyon, you can take the fire road along the ridge, that does some significant climbing but then is up and down (though I much prefer the singletrack).