From an ENSP alum -
Environmental and Energy Study Institute (EESI) is looking for interns and their open positions are paid! Additionally, EESI targets undergrads (rather than grad students) for their internships, which definitely can be hard to find in the DC area.
There are three openings and they will likely be remote internships. I previously worked with one of EESI's Senior Policy Associates and keep up with their activities; they have excellent briefings and webinars. My contact let me know that EESI is actively looking to lock in interviews from students. The organization is small so interns usually get a lot of exposure. From my knowledge, I think that candidates who have strong writing skills and have knowledge of current environmental policy issues nationally would do well in the interview process.