We recently had a 90th for my grandfather. It was on an outside covered porch, open house kind of thing. We kept decorations very simple. One reason was time, we all traveled in, and the other was we didn't want anything that might cause someone to trip or bump into something since a lot of the guests were unsteady.
We ended up with a simple buffet and cake. decorations were balloons (mostly outside but one table topper) and tablecloths. we had printed napkins. The one thing I spent more on was the balloons, we paid for them so no one knew what I spent. I did buy a big 9 and 0 for the yard that were 10$ each and bought a ton of helium latex to tie along the fence. We also had some large posters with pictures, kind of a this is your life kind of thing.
Most of the guests were just happy to be together and could of cared less about decorations, although the family members really liked the pictures. I recommend starting as early as you can if you buy number type things. The stores did not care many of the upper birthdays, we had to go to multiple stores to find what we needed. And the pictures were a lot of work and really needed more time than they got.
It was really fun and I am really glad we did it. My grandfather was surprised at how many people came (I wasn't because he is just so great) It was so fun just to be together. I hope you have as good a time as we did