"This paper, combined with some glue" — singular or plural?

This paper, combined with some glue, makes for an afternoon of art projects.

This is the correct version because you've necessarily put the the "combined with glue" between parenthetical commas. The agreement is then between the subject "this paper" and the verb "makes."

On the other hand you could also write "The paper and glue make for an afternoon of art projects." In this case there is no parenthetical clause and paper and glue combine to make a plural subject (they make).

The sentence per se talks about what 'this paper' does:

This paper (…) makes for an afternoon of art projects.

In the context, how it does is secondary:

combined with some glue

which is why that part is parenthesized.

This paper, combined with some glue, makes for an afternoon of art projects.

This verb refers to 'the paper' alone and therefore is in the singular. Note the verb combined, also referring to 'the paper'.

There is no reason for a plural verb unless the intended meaning is different:

This paper, and some glue, make for an afternoon of art projects.

We are now talking about paper and glue together.