If your budget doesn't allow for this, don't worry too much. It's a free event, and people will be grateful to you anyway. Lunch is the most important meal because you'll want to keep people energized and don't want to provide too much distraction.
But if you can, it's awesome to do this:
Usually buying some pastries or donuts on the way to the venue is enough. Order coffee/tea from a catering company or buy some in the store.
Order something fresh and tasty. Plan for ~25% of the food to be a vegetarian or vegan option (some people just prefer veggies even if they are not vegetarians). If you don't have a lot of money, buying bread and a tray of lunch meats and cheeses, plus some peanut butter, at a store so people can make themselves sandwiches is a good option. If your budget allows, remember to ask attendees about dietary restrictions so you can be inclusive in your catering choices!
Buy some water, juice and lemons with paper or plastic cups before the event so you don't have to overpay the catering company for these items. Usually, 1 litre per person per day is sufficient. If you don't have a car, transporting that much water might be a problem. You can ask some of your coaches or other helpers to meet you at a store in the morning of the workshop and help you carry the water to the venue. In case you run out of water or food, it's good to have a helper available who can go to the store for you during the workshop.
Have some fresh fruit or healthy snacks (energy bars!) laying around the venue. You can buy these in the store before the event. We recommend fruit that you don't have to cut - apples, grapes, bananas, strawberries, etc.