If you can't afford a vacation, then you can't afford it. At 7 and 4, your kids will not miss visiting the Grand Canyon or the Smithsonian or whatever.
There are lots of cool things to do within 30 miles of your own place, regardless of where you live. Sometimes you just have to look for them. Kids love creativity, and don't need an Alaskan cruise to enjoy and appreciate it. I still remember the little things my family did with the same fondness as our trip to Meteor Crater.
Camping is a great idea, and why not make a lesson out of it? How cool would your 7-year-old think it was to be able to chop kindling for a campfire with his/her hatchet?
I see lessons in safety of use of a hatchet, pocket knife, how to build a fire, light it, and put it out.
Most kids I know love fishing. Lessons in safety, environmental responsibility, catching, throwing back, and cleaning fish.
Lessons in cooking over an open fire. How to filet and season a fish and not overcook it.
Don't ever forget that kids love to learn! Anything that's new and fascinating will be a vacation to them.