I had my kids wear a school uniform, which may seem ridiculous. But I felt that my kids behavior was strongly related to what they wore. If they went to church in dress-up clothes, they behaved better than if they wore athletic shoes and blue jeans. If they came to school in pajamas, they acted sleepy at their desks. And my first born daughter loved clothes, and would change them several times a day (but not hang them back up) while my younger three wouldn't change their clothes at all, ever! (Ewww). So I told them we'd use a uniform. My oldest rolled her eyes, and said, "yeah, like what?"
I said, "corduroy overalls and red tee shirts". Guess what? That week, there was a HUGE SALE on Oshkosh B'Gosh corduroy overalls!! I got them for $3 / pair, when they were usually more like $25/ pair. I bought tons of pairs. I bought them in too-big sizes for future years, too.
What does this have to do with "first day of school activities"? Well, decide what you want your child to wear for school (some people do school in their pajamas) and then you can get that special first-day of school outfit with pictures.
One other thought about a uniform - you don't get nearly as many annoying questions when you take your kids out in public if they are in a uniform. In just play clothes, everyone will ask you why your kids aren't in school, but if they are wearing a uniform, then strangers just assume they're on a field trip!!! ha-ha.
We used to do a "back to school picnic". We'd take our school books and a lunch to a state park, and school there for our first week. Just to sort of celebrate the fact that we were free to do so, while their PS friends could not.