I think the dining plan is a GREAT deal IF......
You want to eat table service every day or most days.
or if your party is primarily children. I honestly don't think you could go a week at WDW not paying an average of $11 per day, even if you didn't eat TS any of the days.
For us, we would do it so we could take advantage of using the TS restaurants at a price which isn't too much more than what we'd spend anyway. The reason, of our family of 5, 4 of us are now considered adults (me, DH and DS's 12 & 10--in a couple of weeks). As a result, we're paying, let's round up to $40 per day for adult and $11 for DD=$171. Can I spend less than $171 per day for my family for food. Oh yeah. Could I go to a TS each day and do that, no. I'd be paying not too much less than that just to go to a single character meal. And, you can't get less dining plan days than your stay. If it was DH and I, but three under 9, then the amount would be roughly $113 and I'd go for it in a minute.
As far as other ideas for keeping costs down. Here's some of the things that have worked for me before and things I'm trying this time.
In the past: purchase things like glow sticks necklaces and twirly lights, mini fans, pins (from ebay or mousepins online), ponchos, umbrellas, etc. before hand. You will get them much cheaper, and it's things you'll need when you get down there.
Bring your own water bottles. Even if it's the bottled type you purchase beforehand, it's alot cheaper! And better for you. We either purchase or already have water bottle lanyards/holders, making it possible for anyone to use.
Bring your own stroller so you don't have to rent.
Have everyone have their own fanny pack and a stash of little snacky things. It will stave off the munchies and curb the impulse purchases. One fo the things we will often put in the fanny packs are uncrustables, and that way we can either get a later lunch or earlier dinner and sometimes combine them.
Invest in a roll or two of quarters and pennies for the pressed pennies. One thing I've found is that they really get into it, and when they're coming off a ride right into a store, they now tend to go looking for the penney machine and pretty much ignore all the merchandise. (You can steer them that way too.)
For older kids, give them their own money or Disney dollars. Tell them that's it and what will be included (for example, we pay for snacks and food, but they pay for any souvies/candy/extra pictures, etc.) It's amazing how careful they'll be with their own money!
Bring batteries and film from home (nothing like getting caught having to pay for that there!)
We eat breakfast before we go to the park.
We eat most, if not all, TS at lunch time. (Added bonus, eliminates the long lunch time lines at CS's!)
If we have a car, we buy postcards off site. We bring post card stamps from home.
We re-use our autograph books and our penney holders from previous years. We buy bigger sized pens at home, typically from the dollar store, for the characters to use to sign.
We try to eat offsite a couple of times if possible.
Something I'm trying to do this time around:
Each of my children will have a b-day and we'll have x-mas between now and the time we leave. We plan to give each of them Disney Dollars or gift cards, ponchos (already got DD a princess one), a Disney-themed t-shirt, some pins, pens, etc. as x-mas and b-day gifts. Building up the excitement for the trip and we're not having to "double dip".