Something practical that will make them stand out and/or look even more beautiful than they already are. however, be aware that I am not talking about getting jewelry and calling it a day. That is too easy and can even be seen as lacking thought and creativity. .... It must be personal and from the heart as in If you can explain why you got them the specific thing (bc they said something before that wasn't even specifically that thing, but just a gather of information about their preferences on stuff in general) and show that you pay attention to every detail of their likes and dislikes...then that will earn you a bonus point for being a good listener (cuz u know..we love undivided attention on us) and for being sincere, practical, and romantic (crabs like personal touches to add to the sentimental value of a gift)... Something that will appeal to a great memory from the past will cover the Cancer part, but also unique because the Leo in us will want to stand out as usual, or at the very least, for YOU to stand out to them by being a unique gift giver. For this one- definitely its the thought that counts. Don't just buy the most expensive thing you can think of and think we will be genuinely impressed or fall so hard for you.
NOTE: ALSO CHECK ON THEIR VENUS SIGN FOR IT TELLS WHAT THIS PERSON PERCEIVES AS BEAUTIFUL