I have had two blessing ways and been to lots so here are some ideas:
Mum gets belly henna tattooed and all guests get a tattoo on their hands/feet. My friends all loved this and they felt part of those last few weeks with me. (tattoo lasts about ten days or so)
Guests bring a square of fabric for a bunting or quilt.
Guests bring a blessing to stick into a journal.
Bring a bead to make a necklace for mum (I love my necklaces and I will pass these down to my kids)
Guests wrap red string around their wrist and then cut and tie and wear it until the baby is born (reminds them to keep mother in mind)

Hope this helps