The day school starts, we go out for breakfast. We started when my daughter started her first year of preschool. She's beginning middle school this year, so it has been a long-standing tradition.

Some years my kids started on different days so we had breakfast out each of their first days. Last year they started on the same day and this year they will too.
These days, I work at school and I have to be there EARLY (okay, not super early but seems like it after sleeping in till 10 all summer!). My husband comes home from work early (he works midnights and typically doesn't get off till 7am, but I have to be at work at 7:15) and we bump up our breakfast time a bit. After feeding everyone we go to the school(s) and take pictures in front of the marquees.
School here is always a half day the first day. After the kids get out at 1:00 I take them out for ice cream. We sit and eat our cold treats while talking about their day.
So, while we don't do anything 'big', we still have our traditions that make the first day stand out. No other day of the school year do we go out to breakfast on a week day so it has become a really big deal for the kids. We wanted a tradition we knew we could stick with throughout high school and into college if we wanted to.

We'd have to skip the ice cream in college, probably, but we could still manage a special breakfast.
Now... BEFORE school starts, we buy and donate school supplies. I shop for supplies when I find sales so I rarely spend more than $10 buying supplies for my own two (and that is with them getting 'special' things). Since we shop my stash, I let them buy supplies to donate. That thrills them.

We also take a gift to the teacher the first day.