We were very tired, we were very merry— (1)
You know that the speaker's got to be happy when she repeats a line about her merriment three times! Oh – and one of them just happens to be the first line of the poem.
We had gone back and forth all night on the ferry. (2)
Who would've guessed that public transit could be so exciting? The fact that repeated ferry rides could make our speaker so happy makes us think that she's more interested in her companion than she is in the ride itself.
And you ate an apple, and I ate a pear, From a dozen of each we had bought somewhere; (9-10)
The combination of simple actions (eating fruit) and extravagant gestures (buying two dozen pieces of fruit just for kicks) helps to create a sense of spur-of-the-moment happiness in this poem.