I haven't yet checked with her family -- I shall do so tonight, by email -- but I might suggest that any charity local to you which is involved in cancer research, cancer treatment or palliative care would be pleased to receive donations in her memory. Cancer Research UK (formerly The Imperial Cancer Research Fund and The Cancer Research Campaign), Marie Curie Cancer Care and the Macmillan Nurses are all organisation worth googling for in the UK. Similar organisations exist in the USA and Canada. Any one of a number of local hospices, and a couple of hospice umbrella organisations, would be pleased to have your money. One of the people asking is deeply involved in the hospice movement, and I admire her for the work she does: she'll have colleagues working in places local to you who would value your support.
If you're a taxpayer, there may be techniques provided by your government to ensure that your donation reaches its target accompanied by any tax which you might otherwise have had to pay: in the UK, google for "gift aid", or fill out the appropriate sections of the screen if you're donating online. Similar techniques will exist in other jurisdictions.
If these issues don't enthuse you, there will be others which do. Contribute time, energy and money to causes which you feel are going to help you to honour Susan's memory. Wherever you choose to give it, whatever you choose to give will be gratefully received and faithfully applied.