Merry is usually the wiser, more thoughtful hobbit of the inseparable Merry/Pippin duo, and he certainly has his wise, thoughtful moments in a film which mostly… sees him hanging out on the top of a walking tree.
When they're carried by the orcs toward Isengard, he dislodges his elven broach and spits it on the ground. When Aragorn finds it he says, "not idly do the leaves of Lorien fall," knowing that the hobbits are leaving a trail to help their potential trackers.
But for all Merry's cunning, he's unable to break the stubbornness of Treebeard and the ents. Merry knows what's at stake here. He sees the ents as a crucial element in the war and knows that his friends "cannot fight this war on their own." When the ents decide not to take action, Merry pleads with the, "but you're part of this world, aren't you?!" but to no avail. When Pippin begins to lose hope too, wondering if the world is too big for two small hobbits to make a difference and if they shouldn't just go home, Merry counters,
MERRY: […] the fires of Isengard will spread and the woods of Tuckborough and Buckland will burn. And—and all that was once green and good in this world will be gone. There won't be a Shire, Pippin.
And so Pippin is inspired to try one last time to persuade the ents at the risk of their own lives—and it actually works. He shows the ents the destruction of the forests nears Isengard, knowing that sometimes things have to get super-personal before you're willing to risk your hide helping others. And bingo: the ents go from mild to wild in a matter of seconds—it's revenge-time for the tree-people, thanks to one very crafty little hobbit.
Now in Isengard, they're even farther away from the comfort of home. But hey, at least they found a flooded storeroom to pilfer. There's nothing like some good ol' Longbottom leaf to relax after a hard day of sacking Orthanc.