I took a day trip to BRW in September two years ago and found it well worth it. I don't think it would be nearly as much fun in March though.

Sounds like you are traveling in mid March. Be sure to visit the ice sculptures from the Ice Art Championships in Fairbanks when you are there. Visiting in both daytime and at night is great - different perspective with the lights on them.

Not sure how much time you are allowing yourselves in FAI for the Northern Lights but I would plan on 5-10 days minimum to give you the best chance of seeing the Northern Lights.

If it weren't the evening in winter, I would say to rent a car and drive along Turnagain Arm south of Anchorage. But that time of year and that time of day is very tough. I would probably just wait at the airport.