You've already received some great ideas; here are a few more to add to the pot...
40 Lifesaver mints (they come individually wrapped)
40 Bobby pins or hair pins
40 cotton balls
40 disposable razors
A basket filled with 40 snacks (various kinds of chips, snack crackers, individual size cheeze, popcorn, nuts, the 100 calorie pkgs. of cookies, etc.) or 40 pamper items (q-tips, nail polish, a clear coat polish, polish remover, an aroma therapy candle, a CD with some easy listening jazz, a bath splash, lotion, loofah sponge, moist towelettes, a tea bag, emory board, cuticle stick, some baby oil, fragrance sampler, tissue, shower sponge, body powder, buff puff, disposable razor, chap stick, tweezers, nail file, etc.)
The Dollar Tree has lots of these items for $1 each and you can put two of some of the items that you might need more than one of. Dollar Tree also has baskets and the shrink bags that you can put around it and then shrink to fit with a hair dryer so it looks polished and really professionally put together.
Don't forget to include a list of the 40 reasons why she should pamper herself...and you can have some fun with that...throwing in "you deserve it" on the list several times.
1) Because you're a great friend
2) Because you deserve it
3) Because you're always thinking of others
4) Because it's your special day
5) Because you deserve it
6) Because you're a great daughter
7) Because you never take enough time out for yourself
8) Because at 40 more maintenance is a must
9) Because beauty like yours is worth celebrating
10) Because you deserve it.
yada-yada
HAVE FUN!