Go to the local Salvation Army together and find the silliest outfit you can get to $1. Just trying on the hats can be good for 30 minutes of laughing.
Do you have a local free zone? Go watch the penguins for a while. Or feed the seals.
A friend of mine was once going through a bad week. I picked her up in my car, found an empty parking lot, and drove around in tight circles as fast as I could until she was laughing hard enough to cry.
I was once having a REALLY REALLY bad time one week and - while I was in my bedroom being all miserable by myself - my flatmate ran me a nice deep bath with OODLES of bubbles, lit some candles and placed them round the bath and put a HUGE glass of wine on the side along with some huge fluffy towels and a small flower in a glass.
I soaked for about two hours just thinking about what a wonderful friend I have!
Pack the kids up and take everyone for a hike on a trail. Take along a picnic, with everyone carrying their own food and water in a little backpack or something. Make it an all day event. Sure you will get hot, sticky, sweaty and the bugs will bother you, etc, but there is something about hiking that clears your mind and makes you happy to be experiencing something that is bigger than you.
Take your friend to see whatever festivities for July 4th are going on in your area – not just fireworks, but parades, War for Independence re-enactments, free concerts in the park (usually followed by fireworks), etc.