Totally agree about taking the backpack. I go solo with 3 small children, and really put the BP to good use!
In my vast experience (have gotten this down to an art!!), be sure to pack a bag large enough for your whole BP in case you get caught in a downpour, and I also keep things I definitely don't want to get wet inside the BP (autograph books, camera,...)in individual bags.
Last year I finally discovered a great item to help me with the problem of hauling this heavy backpack between parks (where I attach to stroller with those locking ties). I take one of those lightweight, folding luggage carts, put my small cooler with water, etc... on bottom, then the heavier BP on top, and a small light one to take on rides with things not okay to leave on stroller, and strap them on. This worked out great! I really hate that backpack on my back all the time, and with 3 kids our cargo can get pretty extensive!!
P.S. If you use the locking plastic ties, don't forget a small pair of cutters to remove!!