If you can remember the color of her toenails 1st time you met her,buy her flowers matching the same.....although if she was wearing wellies you be out of luck there
If you met her in restaurant or coffee shop make up a personalized card with tea-bag or coffee bean and say its a memento of first time you met.
Or give her a jar full with all things you have seen and experienced with her in last few weeks i.e.....tissues shared,straws shared,cinema & restaurant receipts,if not else she could claim tax back on some receipts.
Lastly,personalized presents are the best .......mainly because the woman is mostly grateful that you just are able to remember her name and that's a big achievement for most men.
But mostly listen to what she has said and base your present on that,Women appreciate that you listen to them or even pretend to.
And wear a badge saying weirdo stalker,,,who the fuck keeps dirty tissues and used teabags sounds like an episode of criminal minds