Jacket potatoes.

pre roast the potatoes.  prepare fillings/toppings in advance - keep potatoes in a warming station, top with fillings, provide a fork and move on to next customer.

The key is to figure out toppings and fillings - but think of the pre baked potato as the ultimate edible vessel - search jacket potato (UK); can be done as fast food/cart food.

Also gluten free- peanut free, vegitarian and vegan friendly, depending upon toppings/fillings.

Shish kebabs.  It would require a bit of prep, but only minimal cutting and not a bunch of chopping.

Subway makes those really long subs, but I'm not sure if that's the type of thing you're looking for.

Great ideas, the shish kebabs could easily go with the potatoes for the non vegetarian...

You can do veggie ones too.  Tofu or just veggies & mushrooms.

Fruit shiskebabs are good too!

Will there be any fresh fruit in your area when the event is on?  Great fresh fruit is wonderful I think!  A juicy peach, some plums, little cone of strawberries?  Maybe yogurt to go with? 

The GO project is a fantastic experience!  Birthstone's daughter did it a few years ago - you might to send her a wondermail if you have questions. 

I like the potatoes idea!

The thing about selling food at public events - there may possibly be public health regulations that have to be factored in.  I suppose if it's a private club it might be different - but then sometimes they have vendor contracts that have to be considered.  Complicated these days. 

I was just going to comment about food service laws. Especially with cooked food temperature control.......

Freezies to sell on a hot day. Frozen bottles of water, good for a hot day.

But again, most tournaments have vendors.

What about making up banners, those little things that look like triangles on sticks. Print them with the logo of the event they are going to and seek those. Hand out literrature? Would need permission

Or what about putting out flyers and doing grass, garden work for money.

A young woman up the street from us is doing an overseas 'volunteer' trip this summer & had to fundraise also.  She did a direct appeal - a letter in our mailboxes, letting us know what she's up to, and inviting support.  In return for donations, she gifted us with a little packet of beautiful handpainted note cards, which was thoughtful.   She's someone we've watched grow up, and a few folks were happy to provide support. 

Freezys might not fall under "health food", I am afraid and making them from fresh fruit might get too expensive.
Not sure about health law- people here do BBQ fundraisers all the time...
We will ask people from the congregation as well.
My son has a lawn mowing summer job with the neighbours and will put his income towards the cost.
As far as I know, they don't have any vendors, so there is a market. Now keep in mind, our population is around 3000, so it's not the US Open.
I think the potatoes and maybe some fruit kabobs or watermelon by the slice would work.

We have done concessions for cross country ski races.

Bowls of chili and ceasar salad and a bun.

Pop or juice boxes

and yes often small bags of chips.

Depending on facilities you have available-ie stove and/or fridge.

If you have freezer available ice cream cones!

mmmm - watermelon by the slice sounds mouthwatering at a hot tennis tourney!