Bells work great and are polite. If you can't fit a bell on your bike, I think you should stay off of multi-use paths. I found one that has screws on both sides. To get them to fit my dopped handlebars, I had to buy longer screws at the hardware store, but now the bell fits just fine.

I've also had luck by saying "I'm on your left" or "I'm passing on your left." All these take time and require you to slow down, but those are just disadvantages of using a multi-use path.