If anyone in your party must have coke or whatever soda, a mug is definitely worth it. Also, we picked up a cheap styrofoam cooler and drinks at a store off-property, which worked well.
As far as souvenirs...if you have DC, and I think AmEx does it too (not sure about DVC), you can get 10% off purchases from World of Disney and other stores in DD. Sometimes they have the same merchandise as the parks, so we usually go there first, before buying things in the parks.
My first trip to WDW I bought tons of souvenirs. Second trip I bought some, mainly because it was the 25th Anniversary, and I got the special commemorative things. Most of them are now collecting dust, and I don't appreciate them as much as I should. So what did I do on my third trip? Before I bought something big and unpractical, I would ask myself, "is this going to sit there collecting dust?" And before I bought a t-shirt I would ask, "do I really need another t-shirt?" I limit myself to just 1 or 2 shirts per trip (unless I find an excellent deal...found $6 t-shirts at AK!!). As far as expensive things go, if they have a lot and you can return to the store later, sleep on it. If you still want it the next day (or whenver you return), then go back and consider making the purchase.
I also love to collect things, but decided on smaller (and cheaper) things like pins and pressed pennies (I used to collect shot glasses...dust collectors!). Now they have those neat pressed penny books, and they're relatively cheap souvenirs. For my pins, when I get home, I pin them all on a piece of felt.
I have one friend who travelled to WDW with us, and although she could have afforded to shop well, she doesn't buy ANYTHING unless it's on sale!! That might be a little harsh!