We LOVE bubbles at MOP. They work for all ages, can be used inside or outside, and entertain large groups. They are quiet, inexpensive, easy to set up, and relatively mess free. What more can you ask for?
A staple at the school - and one of my personal go-to birthday presents - is a "commercial grade" bubble machine. This one on Amazon is great! Big things I look for are the ability to plug it in so you don't have to mess with batteries and a metal exterior for durability.
Our favorite bubble machine activity is to give the kids plastic fly swatters to pop the bubbles. It's a great way to practice hand-eye coordination and you can work on counting by having a race to pop a certain number of bubbles.
When we first opened MOP, we had a bubble booth at Ridgeland KidFest to tell people about the school. The adults had just as much fun playing as the kids!