We quit going a few years ago. Several years ago, you used to be able to find things in the shops there that you could not find in the parks. Now, pretty much all the merchandise you can find there you can also find in the parks. As far as the
lego store, we have one of those in our mall at home- Not quite as big, but DD would much rather be in the parks (and she is a huge shopper). We find it is typically a hassle to get there and back by bus as it is about 20 minutes from most of the WDW resorts where we stay, and it is typically crowded, with nothing to purchase that we consider unique. For us, our time is much better spent in the parks-- or even at the resort pool.
We have not done Disney Quest as DD was younger last time we went to DTD, but Disney Quest is getting ready to permanently close anyway. I'd rather be in the parks than playing video games anyway.
One other thing to think about is you said you were thinking about doing DTD the last day before your flight? Will you have already checked your bags at the resort? You wouldn't be able to pack whatever you bought. You could put it in a carry on if you had room. Also, if there is a shirt, jacket, kid's outfit or anything you saw in the parks you want that is sized- don't wait assuming you can pick it up at DTD. As sure as you wait, DTD will be out of your size- I've made that mistake before. Don't believe them when they tell you they are completely out of it either and don't know when they will get it again (at any WDW store whether it be parks, resort or DTD), especially if you are looking at the item in the evening hours--- they restock at night. 90% of the time when I've been told that, the item has been at the store the next morning in the right size-- they just won't tell you that it gets restocked overnight because there is an off chance that specific item has been discontinued and isn't going to be restocked.