Make it affordable. That seems to be the biggest complaint I hear about reunions. At 10 years, people are startiing families, have only been working a few years if they went to college and the expense of paying a LOT for the reunion can be a lot.
At our 10 year, they used posterboard (decorated it) and did a top ten list of songs, movies, albums, etc. from the year we graduated. They had a lot of "memorabila" there and they had a memorial to the 3 members we had lost since graduation which was nice as I didn't know about 2 of them.
The centerpieces on the table were easy too. Cut a square about 6 inches. on top of that glue a strip of paper into a circle (you have just made a graduation cap, but upside down) then put fake flowers in your school colors (ours were blue and white) in the center and viola! easy.
Have a DJ and make sure he will only play songs from the 4 years you were in school. Those songs are the ones everyone remembers fondly and ties to important events in their highschool lives.
Invite the teachers. We loved seeing some of our favorite teachers and the principal again and the teachers love to see what their students have done with their lives. And from our experience, they don't stay long.
We also made nametags from the yearbook using the senior picture. Just copied the photos, glued them onto nametags, added each persons name. It helped when it came time to recognize people as there were a few that had changed considerably.
We have had a 10, 15 and a 20. Were to have a 25th last year, but the classmate that has done all of our reunions died suddenly and no one was able to carry on, so we didn't have one. Not sure if I would go back. My husband graduated a year ahead of me, we went to his 10th and he refused to go to anymore. Said he sees who he wants to see. We only live about 35 miles from where we graduated and I do see people from time to time. But I had a blast at the 15 and 20 reunions. Most of the cliques had disappeared and everyone was much more comfortable with their place in life at the 15th. At the 10th everyone was still trying to impress everyone else, but 5 years later, it wasn't so bad and by 20 it was even better. And that one was very relaxed BBQ with DJ and danicing later.
Oh, for the 10th we had a picnic for the families in the afternoon and then the evening dinner was just for adults. The picnic was nice because those with kids got to show them off a little bit and it was a more relaxed atmosphere than the dress up part.
Your local newspaper is a good resource and should do this for free. We would give them the list of names that we hadn't been able to find (I worked on the committee for the first one) and ask them to print:
Do you know where these people are? The Class of 1980 is looking for them.
And people around town (relatives or friends that weren't in our class) could call a phone number and tell us where these people went. We found quite a few that way.
We also started on ours 1 year before as finding everyone is the biggest challenge. And we sent out save the date announcements about 6 months in advance because people out of town have to have time to make arrangements especially if flying or getting time off work to make the trip.