Congrats on the twins! :)
Sounds like you are already well on your way to a great party and also some neat goodie bags...you don't have to load 'em up to the max, just a few things they will enjoy. The star sunglasses sound great! :) Instead of candy, you could put a mini box of raisins in each bag (we have done that and kids love them!).
I agree with the mom who suggested bubbles for blowing - that's a classic summertime fun thing and kids of all ages (and adults!) enjoy it!
I also second the idea of the crayons/inexpensive coloring books. This can work as a "goodie" item AND an activity for during the party. In fact, you could make your own coloring books to really keep costs down; we just attended a birthday party with a character theme where the mom had made a small coloring book by printing out a few character coloring sheets front/back (only about 4 papers total, printed both sides) and attaching them together with blue tape (may have glued or stapled first and then covered with blue tape for safety, I can't tell). These were given to the kids to color at the party at the beginning (as an activity) but then they could take them home and finish up later. We got there late so my daughter did all her coloring at home. :)
Such a coloring book is very inexpensive to make if you have a computer and printer at home. You could seek out coloring sheets of flags, fireworks, Independence Day themes, or just summery pictures of animals, the outdoors, etc. My daughter is 2 1/2 and really liked the coloring book a lot - she has heavily colored every page and worked on it for a few days following the party.
Another thing that was in the goodie bag from this latest party we went to was a pair of tiny maracas, and my daughter has gotten a lot of fun out of those. :) Kids of all ages enjoy these and the noise isn't too objectionable like some other "noisemakers" can be. I think the mom found them at a party store, or possibly ordered them from Birthday Express.
One neat thing to do at the 4th of July foodwise is to have strawberries and blueberries - they look good and taste good too. Kids usually all like these too and they are healthy rather than having cookies, candy, etc. as dessert items.
Have fun with your son's party! It will be great! And at this age, repeating a theme is no biggie - we did Veggie Tales last year for my daughter's 2nd birthday, and plan to do it again this year - she still loves them so she will be happy.