Climb the sentinel was a mission in The Precursor Legacy that Jak and Daxter completed to obtain a power cell. It is an unassigned mission where Jak found a power cell after exploring the stone sentinels at the end of the beach.
At the far end of Sentinel Beach you can find three big stone sentinels. These can be reached by crossing the bay or walking around and over the higher area on your right when you enter Sentinel Beach. Be careful of the two babaks and a lone sand worm, as well as a group of lurker crabs that roam the beach prior to entering the area with the sentinels. The middle sentinel is the one holding a power cell at the top, simply ascend the stone steps and collect it.