The poem is not about the flowers
The first three stanzas are reportage: wandering, seeing ten thousand daffodils along a bay, comparing them to the Milky Way, and being unable to calculate what the show was worth. The fourth stanza leaves the outdoors entirely. The flowers are only ever valuable in retrospect, and the poem says so plainly.
This is worth noticing because it inverts the usual pastoral claim, which is that nature is good for you when you are in it. Wordsworth's version is that nature is good for you at a distance, when memory does the work — which is a much lonelier proposition.
Vacant is the most interesting word in the poem
In vacant or in pensive mood. Vacant means empty, unoccupied, and it is not a flattering description of one's own inner life. The poem needs the mind to be empty before the daffodils can flash on the inward eye. The bliss of solitude is not presented as a reward for sensitivity; it is presented as what happens when there is nothing else in the room.
Read that way, the closing two lines are less a celebration than a report of a mechanism. The heart fills with pleasure and dances with the daffodils — but the dancing is happening indoors, alone, and to a memory. The word that carries the whole ambiguity is with: the heart dances alongside the flowers, not among them.
The inward eye is a claim about memory, and it is contested
Wordsworth's phrase became a slogan of the Romantic movement — the idea that the mind's eye can reproduce experience well enough to replace it. His own note on the poem credits his wife Mary for the phrase inward eye, which is a small reminder that this most personal of lines was written by two people.
The sceptical reading is available too, and modern criticism usually takes it: the speaker's rapture is nostalgia, and nostalgia is what you have instead of the thing. The poem is honest enough to leave both readings standing, because it names the mood as vacant before it names the reward as bliss.
What to steal from it
Steal the structure: an outdoor scene in present-tense action, followed by an indoor scene in habitual time that reveals the scene was really about remembering. The two-part shape, with the second part much shorter than the first, gives you the pleasure of the description and then the argument about it, in that order.