I do the same thing. You might try a bowling alley. They are usually both simple and inexpensive. Some cities also have hidden treasures. Great locations for parties. Check out your local Chamber of Commerce.
My 6 year old son's favorite parties have been the ones thrown in our own backyard. Here are some at home/backyard ideas:
Movie party - Do it late in the afternoon/early evening with hot dogs, nachos, popcorn candy,etc. They could watch your son's favorite movie or screen a New Feature, play movie trivia games (age appropriate of course)
Carnival - Make Carnival games out of things you have around your house. Bowling, rubber duck pool, basketball shoot, etc. Give each kid tickets when they play a game(win or lose) and at the end of the party they turn their tickets in for a treat bag or set up a table of different little prizes and let them choose a couple. Food is simple think of what you eat at a carnival and there you have it. Also you can order from Oriental trading a "how to" book for balloon animals. Basic balloon shapes are pretty simple, you can ask a friend or family member if they can help out with this.
Spring theme - The kids can do a scavenger hunt for bugs, specific flowers/plants, and other earth related items. When they find everything, the can make a craft out of it (not the bugs of course). Maybe have bug boxes on display for the kids to put the bugs in so they can all look at them. For a cake make the Oreo Dirt Cake in a sand pail with gummy worms.
Harry Potter - this was the most elaborate yet least expensive party we have ever had - Most of the ideas for this party I got from www.birthdaypartyideas.com The kids glued jewels onto plastic cups for transfiguration, made Gak for Potions, sat on gold balloons with a small plastic dragon figure (wrapped in a lot of plastic wrap) inside to hatch the dragon for Care of Magical Creatures, Had a table of themed candy and figures for the kids to visit Honey Dukes at the end for their goodie bags. We took a witch's hat and had a sorting ceremony.
I hope I have helped a little. Good Luck.