We've done one of these for the last several years and do not charge the vendors to come. One thing we did for the last two years that was very successful was to set up a scavenger hunt of sorts--each vendor submitted one question/activity to us in advance, and the kids had to find the answers or do each activity on the sheet to be entered in a drawing for prizes. This helped ensure that the kids were engaged in some way at each booth. So for the local rec center, they might have to go and try an exercise ball or do 10 jumping jacks to get their sheet checked off; at the local farmer's market booth, they had to find the answer to the question "when does the market open for this season?" Every booth had one question or activity.
Dentists/orthodontists are always happy to come. We also have the local fire station bring out a truck or two, and they do a "stop/drop/roll" exercise and have their equipment on display. The sheriff's office brings a kid's fingerprinting machine and prints child ID cards (by far the most popular booth). The Lion's Club sends out a van to do hearing/vision screening. A local hospital always has a booth. Since we are near train tracks, we have a model train group come out and set up their trains, and they do a presentation on rail safety. Some of the other booths have been provided by a poison control center, an organ transplant organization, the Red Cross, a vet, an allergist, Mothers Against Drunk Driving, and the local government's emergency/disaster preparedness office. The police also did a child car safety seat check one year (check safekids.org for this), and we also talked to them about doing a bike rodeo. We also had a Girl Scout troop that did a whole presentation on calcium (they did it as a badge project from Cabot Cheese)--they gave out Cabot Cheese samples, pencils from the local dairy trade association, and took "milk mustache" photographs (done with melted ice cream...). Another GS troop one year had a booth on food allergies--they had a display of GS cookie boxes, and you had to guess which ones were safe to eat if you were allergic to peanuts, or wheat, or eggs, etc. A chiropractor came one year and did an evaluation of kids' backpacks' weights. I want to say that an orthopedist's office brought in some x-rays of broken bones and did something related to sports injuries. Basically, if you have a medical office park near you, take a look at their directory and see what kinds of medical practices you have there and how they can relate to kids, and you'll come up with all kinds of ideas.
Almost all the vendors brought giveaways of some sort, so we provided bags for people to carry stuff around. Also, you can contact different trade associations and they may be willing to send you info that you can give out to parents, even if they cannot be there.
In addition to the giveaways that went to just about everyone, we asked each vendor if they would contribute one larger item that we could use as a doorprize. Not everyone did, but we got some pretty nice things donated--a one month membership to a gym, an electric toothbrush, Starbucks cards, etc.