A word that means "small gift"?

little something

Sorry I couldn't buy you anything, but here's a little something to spend.

Macmillan defines this phrase as:

a present that you give someone to thank them, that is not very expensive. [Example] It’s just a little something to show how much we appreciate your help.

My aunt used this phrase all the time especially when she sent me money enclosed in a letter when I was in college.

Just sending you a little something! Spend it foolishly!

And there is nothing wrong with small gift or token gift, although token means, according to Collins

You use token to describe things or actions which are small or unimportant but are meant to show particular intentions or feelings which may not be sincere.

A token gift can be a small gift, even one of no monetary value, but sincere, despite the qualifier above. (Note that may here means sometimes, not always.) One example from Collins is:

We presented both our guests with token gifts of appreciation. Hebblethwaite, Peter Paul VI - The First Modern Pope (1993)

Token might be the word you're looking for.

Sorry I couldn't buy you anything, but here's a token of ten dollars to spend.

or...

Sorry I couldn't buy you anything, but here's a token amount of ten dollars to spend.

Google defines has two definitions that I believe work well together to sum up what you're trying to say:

A voucher ... typically one given as a gift or offered as part of a promotional offer.

And...

Done for the sake of appearances or as a symbolic gesture.

Together, this word can be used colloquially to mean a small gift given less for the value of the gift itself, and more for the thought behind it.

From regional American English dialect: (East, Southeast and Central Plains) :

Sorry I couldn't buy you anything, but here's a sirsee of ten dollars to spend.

or

"Sorry I missed your birthday so i got this sirsee for you."

Sirsee, from Room Mom blog and A Way with Words

Spelling variations– circe, circi, surcy, surcee

Definition– (n) word used in the south to mean a small, thoughtful gift.

The Dictionary of American Regional English (paywall) has a listing of "sirsee" (variously spelled "circe," "circi," "surcy"). East and South east U.S.: NC, SC, GA, and PA, as well as two reports from Buffalo, NY and Oklahoma. The etymology is uncertain, but one plausible source is the Scot/Irish verb "sussie," meaning "to take trouble, to care, to bother oneself."

Sorry I couldn't buy you anything, but here's a soupçon of ten dollars to spend.