Ask HN: What do you look for in an internship?

I am anticipating a number of internship offers from a variety of companies for software developer roles.

1. What are some important (positive) things to look for?

2. What are some things (negative) to watch out for?

3. Long internship or short term (get to come back to school and graduate a year sooner vs take an extra year overall to graduate but with more experience)?

4. Big company or small startup? All in Canada if that makes a difference.

5. Going back to (4) graduate sooner... take extra year to graduate and maybe fit in another internship next summer? Or is it more valuable career-wise to graduate right after this internship?