Yeah I have a bag in the car with 1 nappy for each child and spare clothes, and a change of shirt for me. In my nappy bag (which is just an oversized handbag) I carry (for 2 children.... 1 3yr old and 1 6mth old):

2 x nappies for the 6mth old
1 x pull up
1 sml pack wipes
2 plastic nappy bags
all this is wrapped in a fold out change mat which is very compact
1 wrap
1 dummy
1 change of clothes for the refluxy 6month old
1 bib
2 snacks for the toddler or lunch if needed and a water bottle for her.

Thats it... when Jovie was having issues with breast refusal I would take a bottle with water & a container with formula in it. Having 2 I didn't want to be carrying around heaps of stuff... if I needed it I could find it in the shops.

I don't like carrying around heaps of stuff, and I guess when we traveled to the US with an 18 month old we had to learn about coping with not having something on hand while on the plane etc.... I don't mind coping because I'm not often out of the house for 8 hours. Of course that didn't help when Matilda was a baby & in the sling & projectile vomited down my shirt and pants and even into my shoes... that time we had to just go home. A change of shirt wouldn't help