As someone who suffered (and I do mean suffered) as a child with seasonal allergies, I'd say, whatever you do, don't do nothing. We are so fortunate to have a whole new generation of allergy medications that allow normal functioning. When I was a teenager, the entire spring season was a miserable blur. The only medication that was available to give relief was benedryl, and you will notice that many OTC "sleep aids" are just benedryl. Most people will just sleep when they take it. It's fine at night, but not practical for many people during the day.
Anyway, there are wonderful, safe alternatives these days. My son's been taking Zyrtec with pretty good results. I noticed other people mentioned Nasonex, which I take and find to be amazing, and Singulair, which I also take during the worst part of the allergy season (although not sure it's safe for kids). Today I spent the whole day outside gardening and at my son's Tball game. If it wasn't for the meds on the market now, I would have literally been in bed the entire weekend with the windows all closed!
Anyway, whatever you try, give the dose at night only, for the first couple of days, and see if that brings some relief. A lot of the coughing and post-nasal sinus stuff will happen at night anyway, so even treating at night can bring relief during the day. If you suspect seasonal spring allergies, if she spends the whole day outside make sure she takes a bath to wash all pollen off (hair too) before bed. Keep her bedroom windows closed at night.
Also, check out her classroom. I was in my son's class last week and one of the kids is just red-eyed and tearing with horrible allergies, and the teacher had a fan in the window with a flowering tree outside. Of course since the teacher doesn't have allergies, she didn't realize what a bad set up it was. (Avoid fans in your daughter's room too - they just stir up any dust in the air and move pollen around.)