My LO is the same - she'll do a 45 min nap in the car or frontpack (we don't have a buggy), but no more. So I generally stick to half day outings - that way I know she should get one good nap in her cot, and she can handle the other one being short, maybe with an early bedtime. If we have to be out for the whole day, I aim for three short naps! Do you have a travel cot? That might be another option, if you get your LO used to sleeping in that when out and about.