As a 15 year old boy, I think just going to the movies is fun. If you wanna try something different, however, try planning a picnic if its nice!! That's different and fun. Or if your stuck, just ask them. You'll get good ideas from them prob, after all, they are the kids. Hope I helped!
It's a perfect time for some major spring house cleaning. Get the walls washed and all those pesky little chores taken care of before summer vacation. Doing these things now frees them up during summer vacation so they can concentrate on summer jobs.
Not everything is a National Park. Some are historic sites - like presidential childhood homes. Odds are there is something you can schedule a day trip for.
The younger kids can participate in something called the Junior Ranger's Program. They get a workbook from the Ranger's station and answer questions, Once enough questions are answered, they return the workbook back to the Ranger's station and get a plastic badge for their accomplishment.