-( Story related, cannot be missed )-
*While this is "flagged" as possibly story related, the real note is that... you could scrape by without having all 101 Power Cells early on. It would be in your best interest to avoid back-tracking at the end to get this for the final ending.
Level: Forbidden Jungle
The fisherman can be found at the bottom of the waterfall and small river under the broken bridge. He will ask you to collect two-hundred pounds of fish. This little mini-game can be a bit frustrating, though. The green fish are worth one pound, while the gold ones are worth five. Meanwhile, the poisonous pink eels will cause an instant end to the mini-game, so avoid them as best as you can. Make sure you catch all of the fish you can, as if you miss twenty pounds, he will also call the mini-game off.
Completion of this mini-game will reward you both a Power Cell and access to Misty Island using the hoverboat in Sandover Village. There are several rounds to the mini-game, increasing in speed and difficulty the further you go in. It is more beneficial if you can avoid a few green in lieu of the gold fish, as one takes a lot longer to amount to twenty than five does. ;) Yay for mathematics!