A little something for mum, dad and baby would be nice. Maybe try find out if there is anything she hasn't bought yet?
Chocolate, wine, bath things, nice lip balm or hand cream, homemade biscuits or posh coffee(handy to offer to all the visitors), new pyjamas, a pretty bracelet, maybe a voucher for a massage/pedicure?
Maybe a pack of nappies, wipes, cotton wool, baby shampoo etc. A baby grow or a cute outfit, a teether or small teddy. (BTW, I found that nearly everybody brought clothes in either newborn or 0-3months, which my boys grew out of nearly before we had a chance to try them all out! So think about buying a few sizes up) Think about blankets, a photo frame or baby book, a little keepsake box.
Beer, a take-away voucher, a book? Some ear plugs for a joke!
This is a lot of effort, but one of my friends made me a canvas picture she made herself of cut-outs from magazines and photos of us throughout the years. Was really thoughtful, and was by far my favourite present, probably just because it was so personal.