My daughter won a prom package two years ago from a big box store, it had perfume, matching earrings & necklace (cubic zircona) and a solid colored clutch (her choice - gold, silver, black or white). It was a nice package.
Tiarras are out - with my daughter's group, doesn't mean they are out nationwide. I would not go with nail polish, most girls go with a herd of other friends to get their nails done prom day, most get their hair done too.
If this is a giveaway package you are putting together I would not put too much in it especially if it is an expense to you - perfume and jewelry are great. Maybe a small gift card at a popular local fast food restaurant for a quick bite between their hair/nails, getting dressed. The clutch was perfect - also these large hard cased wallets are very popular. Gloves have not been popular for the last couple of years - but golly they sure are classy wish you could make them come back.
Lipsticks, lip glosses and breath mints would be nice also. I wouldn't go with makeup because of the sensitivity of so many skin types. But maybe a gift card to a nearby salon (could be used another time as well) would be nice.
The more extravagant & ridiculous expense has been limo's and some expensive, overpriced dinner.
Here in our area of Texas all the proms I am aware of are in the spring - late March to early May.