People applying to intern positions have insanely widely varying skill levels. Some already have some previous experience and/or extensive side projects, some may have just started programming in university and might be "clueless" about many things. So you will want to have some coding challenges or technical interviews (obviously doesn't have to be and in many cases shouldn't be some algorithmic challenge or some other bullshit like that), for assessing if this person has the skills to be able to fulfill the basic requirements of the job.
Assuming you are also looking to hire interns for full-time after, you will want to assess if they are able to learn things, how do they work in a team, and overall how do they fit into the company/culture/industry.