S.H.
You did not say what Island you are going to.
But, in Waikiki, there are SO many "tourist" shops that have small trinkets.
And there are often sales.
Think of a budget that you want to spend, then times that by 18.
That can be a chunk of money.
There are things like:
magnets
pencils
tiki necklaces as you said
erasers
nail files
those flower leis
soaps
stickers
Hawaiian Calendars (which is cheap). My in-laws bought some from "Long's" stores, that were only like $1 a piece and it was a nice Hawaiian photo calendar. It was on sale.
Shell Leis. Which yes, can be on sale for cheap.
The thing is, you gotta buy 18 of these souvenirs. Be prepared to spend about $4-5 bucks, for 1 thing.
Unless, you shop around, and find those flower leis and Tiki necklaces, for less money.
My In-Laws, bought a Tiki figurine, again from a "Long's" store that was about $4.00 for one.
Or, you can buy, Hawaiian chocolates.... in a bag, that is individually wrapped.
And, you don't want to buy something, a souvenir, that will just be thrown away or considered to be "dust collectors." Because, it all costs money.