It helps that it's a day trip and indoors, so no: camp chair, stool or table, cooler, gallon of water, pop-up shade, etc. It helps that Ben is breastfeeding, so no pump, bottles, etc.
My backpack has in it: feast gear for one, diaper supplies (including spare clothes) for one, an alto recorder, documentation for my performance, a bag with embroidery (the tafl) and directions to the event. So far, so good. But Ben will need a pillow to nurse on. Ooookay... I'll tie it to the bottom of the backpack.
Know what would also be nice? Sometimes, I can't find a free seat at a table and it'd be sweet to have a little carpet to stake out some floor. Oh, and a pillow to sit on. Oh, and some kind of pad to lay Ben on in case his freaking car seat wasn't good enough for that purpose.
...I have managed to stop there. The ties on the seat cushion go around the carpet and tie to the backpack straps, then the wool shawl (can be folded into the pad OR used as a nursing drape!) covers the seat cushion and the nursing pillow, tying them together into one package. It looks stupid but it works.
In fact, it works so well that I'm considering throwing the lyre over my shoulder... Bad Teleri! Stop!
*If you don't get it, start watching Spaceballs at 4:37 or so.
**In the worst-case scenario that I get there before anyone else I know who would happily hold the baby or grab a bag from the car for me. I know, I know.