Nate Powell Calvin & Hobbes

Ummm, can anyone tell me which particular strip this is referencing, because I don't "get it"?

Hold the phone. My baby sister has explained this cartoon to me (or at least given me a plausible explanation). This drawing  shows an adult Calvin and Hobbes, 25 years on, looking with wistfulness at the box which was an important component of many of their youthful escapades. In some ways they have outgrown the box (certainly physically in the way that they dwarf it) but it still holds happy memories for them. Now what do you think that might be a metaphor for, hmmm? Beautiful deep piece of work. Makes me want to go and seek out Mr Powell’s work; an artist I had never heard of before in spite of his many accolades. Incidentally, I don’t suppose anyone reading this does not know but just in case – as of this writing (March 1, 2020) this piece of art is in an eBay auction until Friday morning (UK time) with the proceeds going to a very worthy cause, The Cartoon Art Museum in San Francisco.