Initially, we were on a superstrict budget too - hotel and air ate up about 95% of what we had! Now, we've "found" some $, so I was able to loosen up, but here are some of the cost-saving tips I found earlier:
Get the fridge - you're lucky to be bringing that baby! With a fridge, you can bring perishables (like milk for cereal, cream cheese for bagels etc.). That makes breakfast in the room much easier, especially if your kids really need a "normal" start to their day. Also, a fridge lets you keep some lunch/snack items on hand that can cost a fortune in the parks - deli sandwiches, fruit, yogurt, and WATER come to mind - and especially beer or wine, if that's of interest. And if you do splurge on a table service meal or 2, you can take a doggie bag for anything you'd enjoy eating cold - especially dessert. Don't forget a small squishy cooler with a couple ice packs. I've heard the fridge's don't have a freezer, but if you refrigerate a bottle of water or some juice boxes, that should keep the food cool enough until you're ready to eat it.
For CS meals, consider Cosmic Ray's 1/2 rotisserie meal - feeds 2 for $9. Tomorrowland Noodles is reported to be a good deal, as is Main St. Bakery. At Epcot, Sunshine Seasons has an Asian combo 2 entrees/2 sides that can feed 2 and lots of the counters/carts have non-sweet snack items that could be a small meal (like croissants or egg rolls). There are a few other threads with this kind of info, so I won't go into any more detail here.
One thing I agree on - DON'T get the meal plan. When I mapped it out, I could do all our food for 8 days for about $300 including tax (and doing 2 sit-down lunches).
DDP for 2 adults/1 child was over $700 including gratuities. Way more food, sure, but DDP is not a "budget" plan - it's meant to simplify the vacation. There are great reasons to do the DDP, but especially with a car, you can eat much cheaper without it!
And since you have the car, go to MouseSavers.com and check out their stuff. You can order free discount cards for many restaurants in Orlando, find great coupons, and potentially save most of the cost of your car rental!
For souvenirs, we are stockpiling little stuff for a "trinket store" - fun things from the dollar store,
Walmart, Michael's, Target. Each morning, we'll let my DD4 "shop" from the bag of trinkets. That's it for the day. We are also picking up some slightly bigger stuff - a nightgown, a t-shirt, a princess dress (ok, that's the big one), a game, etc. (mostly stuff from disneyshopping.com). We'll leave that in the room when we leave for dinner each night, and when we get back, the "princesses" will have left her a gift.
But, we also know we can't COMPLETELY avoid the gift shops, so we asked the grandparents to lay off the coloring books and tees before the trip, and JUST give her Disney Dollars. When she sees something she really wants, we'll take a picture with the cell phone. This gives her time to really think about what she wants to spend her $ on and it gives Mom & Dad time to help her evaluate the options. The SECOND day we visit that park, she can go get the item(s) we agreed on.
Take advantage of the few freebies at the parks too. AK has a free "Logbook" that you take to 6 Kids Discovery Club stations and get a stamp. Epcot sells passports with the same idea - OR you can make your own (check the Just For Fun forum and look for Creative DISigns). Epcot also has a free souvenir that kids can add to at each Kidcot station. Make an autograph book, or buy one before you get there - it's an activity and a souvenir in one. You can email a picture of yourself to friends/family from Epcot Innoventions (not sure which one).
Before you go, the WDW website has a really cool make-your-own section where you can print out coloring pages, mazes, fun stuff for waiting times - AND there are a couple print-your-own personalized stories (Cinderella's Sleepover, a Goofy story, others?). Go to disney.go.com/magicartist/index.html There are TONS of other sites with coloring pages - print out a bunch, make some books, whip them out when called for!
If your kids can understand and get into it, point out that every picture is a souvenir - and they can help you make a scrapbook when you get back. Let them help you pick out some supplies for the book BEFORE you leave, so they have a visual of what these pictures will become at home. Pick up small scrapbooks for each of them too - a personal book of their own. Even give them disposable cameras if they're old enough.
If you have time and really want to get into the make-your-own stuff, the DISign stuff on this sight is amazing - the personalized t-shirt designs are incredible, and all you have to buy are the t-shirts and some iron-on transfers.
Wow - sorry this is so long. I hope you have a magical - and affordable! - trip!
-Kristin