In the middle of the Ohlone Regional Wilderness sits Murietta Falls, which has the distinction of being the highest water fall in the Bay Area. In the winter and early spring, it's a fine looking waterfall.
The trailhead starts in the southern end of Del Valle Regional Park at 750'. Get an Ohlone Wilderness Regional Trail/Permit at the park entrance, which is the best map you'll ever get for $2 and allows hiking in the Wilderness for a year.
The trail proceeds upward very quickly, reaching 2380' in a little over two miles. The trail then goes down into William's Gulch (1890') for a short respite before heading up the other side to 3550' (this section is called "The Big Burn"). After passing Schlieper Rock it's an easy 1.5 miles to Murietta Falls. . .
The funny thing about Murietta is that most people hike it in the long days of summer. Not only is it twice as hard in the Livermore heat, but the reward for all the effort is a bone dry water fall. That's right, it's drier than you think. But don't worry, because now there is a cache nearby--surely that's enough motivation? In fact, now that you've purchased the Ohlone Wilderness Permit, you can come back and hike the 28 mile Ohlone Wilderness Trail from Del Valle all the way to Mission Peak. It's a Bay Area tradition (for hiking nuts, anyway).
The ammo box should be easy to find. It is shoved between the ground and a rock cropping within viewing distance of the base of the falls (and close to some shade). The trail (fireroad) approaches the falls from the top and jogs around the streams leading to the falls, so once you see this, you'll realize where the falls are (plus you have the coordinates of a nearby location). There are many safe routes down, including a use trail on the left of the falls (looking down). Don't try to climb the rocks!
Be sure to bring plenty of water for this hike in the summer. It's more difficult than the distance suggests, and the trail is exposed most of the way. The hike back to your transportation is not all downhill, either.
If this is your style of cache, be sure to visit its lonelier cousins in the Hardcore Hiker series. Perhaps you'll show up on the list of those who have found caches in the series. Are you hardcore?