How about a pram blanket? It's nice to have some new things for the new baby and actually, I associated blankets with each baby far more than clothes or toys. ie I would have felt funny using DC1's blanket on DC2! (but didn't mind reusing clothes or toys). You could go for something traditional (cotton) or modern (fleece) and you can have the fleece ones personalised with the name.
A really nice brush and comb set is useful and again, it's nice to have a new one for each child.
Have a look at Not on the High street.com for personalised baby gifts. You can get a framed birth details print containing name, weight, etc. which is lovely to keep.
Don't get hand/footprints set, or baby book (often duplicated gift or the mum will have sorted one out herself), or photo album (often goes unusued). Teddies are dust-gatherers unless they are handmade and then they are guilt-inducing dust-gatherers. 
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However a photo frame goes down well (with New Baby/Our Family etc type thing (ideally subtle) unless she's very chic and would never put a family photo in a "our family" frame!
In terms of nappies and wipes etc, it's a nice practical idea for a first baby, but by DC2, she will be used to chucking all that stuff in the trolley so it's not quite as much of a novelty gift to receive for a DC2 
A nice touch is something small for mum. I got some bath salts and a small box of chocs just for me and I reallyreally loved that
Flowers get a little bit in the way so if you were thinking flowers, just a small bunch and bring your own vase unless you think she won't have any.
It's lovely you are so thoughtful.