You will obviously have to write the response or the continuation for yourself, but here are a few features of the poem that you should take into consideration when you craft your assignment. First of all you should notice the very regular rhyme scheme. In each group of six lines there is an ababcc rhyme pattern. You will have to repeat that pattern 2 times when you write your 12 line assignment. You don't need to use the same rhyming sounds in each set of 6 lines.
Another feature of the poem is that it has a very regular meter. These lines are in iambic tetrameter, so you will need to try to follow that meter as well. An iambic foot is one unstressed syllable followed by one stressed syllable. The sound of the line should sound like this: ta DA, ta DA, ta DA, ta DA. Those syllables can be in single syllable words (but that is kind of restrictive), or can carry over several syllables. You can hear the meter of the lines if you read the poem aloud and over stress every other syllable. Try reading the bold words/syllables with more stress:
For OFT, when ON my COUCH I LIE
In VAcant OR in PENsive MOOD
In regards to what you could write about, you could do a lot of things. You could extend the poem by explaining some other feature of nature that also inspired the poet's joy with nature. You could write about some other feature of nature that brought him sorrow or fear to serve as a contrast to the poem above. You could write about a scene that inspires you when you reflect back on it in your quiet moments. The themes expressed in this poem, the power of nature and imagination/memory, are central to the Romantic period, so it would a very safe route to use those as a place to start your response.