We go for a week - but ours goes like this - this for when ds was 2 and a bit.
4 tops/tshirts for dh, same for me. 5 for ds and 2 spare in the car, with spare pants and spare shorts (in case of car sickness). 1 extra pr shorts/trousers/skirt for me and dh, 4 or 5 prs trousers for ds. One extra jumper for dh and me, 2 for ds. 5 sets underwear for all. Nighttime pull-ups for ds. PJs for ds (3 prs - he can normally wear for 2 nights). Waterproofs for all.
Cot mattress (found easier than travel cot) and two sets of bedding. We didn't take a change mat as he had potty trained by 2.2; potty, collapsible step (for reaching sink).
Sandals, wellies, beach shoes for ds(we got some in Dunnes, and another time in Matalan) - sandals and perhaps one other pr shoes for us.
2 drying up cloths, one hand towel for kitchen, 3 bath towels (not sheets) 3 beach towels.
Dog, dog towel, dog duvet, dog bowls, dog food, dog chews.
Suncream, sunhats, sunglasses.
WASHING POWDER - enough for 4 washes scooped into separate bags for ease of use.
We pack quite a few things separately in bags - eg towels - as then you can use the towel to fill gaps, including the footwells in the back of the car - and they'll still be clean when you get there that way.