I know you mention that she doesn't really bother about jewellery but the 18th and 21st are milestones that should be remembered and when we are older I personally believe that you need
something to remember it by, meals, theatre, spa's pamper days are IMO for
other Birthday's, 18,21,40 are all special milestones and need something to show that. (Only my opinion of course)
My brothers and I all had a special piece of engraved jewellery for such Birthday's which we all still wear to this day, we will always have them, they came from our parents at a special time.
The Bradford Exchange, Compton and Woodhouse, many other companies - Do all sorts of items from antimony boxes to jewellery from mother to daughter, a nice keepsake.
Many companies do, throws, blankets, duvets, cushions with photo's you could perhaps have photo's of her from birth to now put on one of these, lovely keepsakes, or make one of your own. Or just have a cushion saying from mother to special daughter on your 18th.
Most of our surnames have a family crest, what about a pendant, ring, with the family crest on it, girls especially change their surname when married so for 18th and 21st Birthday's they are a really nice keepsake too.
There are companies which do framed records with a message plaque, have you a song that means a lot to you both? You could then have a plaque of engraved words to your daughter underneath.
Just get her or make her anything that she can keep at this special time, I agree the baby can be celebrated later, make this her time.