I think it depends how much you go out, will you use the buggy or a sling, do you have a car with you where you can leave a big bag and just carry a small kit etc.
I use one of these sorts of things if I’m without the buggy, fits in my regular acessorize rucksack or carried on its own. Fits wipes, few nappies, change of clothes and an incontinence sheet that we use as disposables change mat.
Folding nappy changer
If I’m with the buggy I have one of these
Buggy organisers with nappies, a mat, a baby gro and wipes in.
If I were bottle feeding I’d like a bag with insulated/waterproof pockets that stay upright and contain leaks. As she gets older & we need more things, we might want different pockets for toys, snacks, waterproof pockets etc and something unisex & comfortable to carry. I’ll get one of those from Amazon for £20-30. Depends how organised you want to be.
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