If there was one big ticket item that I would recommend, it would be microscope. We have had so many great experiences with our microscope since the purchase and I have also really loved getting to see things. As a public school graduate, I would have loved to have the chance to see a bee or strand of hair under the microscope in elementary school. When I had a job after college that involved fixing phones, I got a huge kick out of looking at my finger at 10,000 times magnification.
I recommend getting a microscope that allows you to look at both slides and 3-d objects and we opted for the Thames and Kosmos Biology Tk2 Scope. I did a lot of research before purchasing and wanted to start with one that was affordable since my kids are still young. This scope came with the basics but also materials they can grow into as they learn to prepare slides. For now we are mostly enjoying seeing what things look like and coming up with new items to investigate. Currently on amazon, the scope is around $90, but I paid $65, so there are deals to be had if you do a little googling.
Here are some of the things that were really cool--
Wasp (the stinger is so cool!)
A baby tooth
Black mold on bread (we wore masks to be on the safe side)
I think my favorite so far was the black mold because it was so amazing to see much more than I expected and I can't wait to explore even more.