V.P.
DFW Airport Observatory - Picnic & watch planes takeoff &land
Find cool parks in your area that you've never been to.
Have a scavenger hunt - either natural at a park (find 3 pinecones, an orange leaf, spot a squirrel, etc.) or create one in your own backyard with things you've hidden.
Check your local library & rec center. Coppell Library is having a special activity every afternoon (free movies, The Creature Teacher, etc.)
Go to Bass Pro or Cabela's and explore. They have lots of museum quality animal diplays and huge aqariums (check fish feeding times online).
Have the kids help you make a plate of cookies and bring them to the FIRE STATION. If there isn't an emergency, the fire fighters will give them a private tour of the fire trucks!
Grapevine mills mall - If you take the kids before they open (I think the mall opens at 8 and the stores open at 10), you get some excercise walking around, and the kids love going up and down the ramps.
We live in Flower Mound and www.familyeguide.com has featured lots of activities in our area (Lewisville, Flower Mound, Highland Village, etc.) that you can check out. Even if you don't want to drive up here, they may give you inspiration for other activities close to your home. They'll also be featuring activities during spring break (which is 3/16 - 3/20 in LISD).
Good luck, have fun, and relax! You don't have to schedule too much, just let the kids make their own fun sometimes.