I refuse to give vouchers. I'm also avoiding cash as their disposable income is currently greater than mine on account of them not paying keep (students, but living at home).
I'm fed up finding unused vouchers. Also, where we live, vouchers devalue at the rate of about a fiver a month, so a 20 euro voucher is defunct after four months, by which stage they've barely taken it out of the envelope 
So I'm buying random crap they wouldn't otherwise bother with. Boxers/knickers, selection boxes, socks, possibly perfume/aftershave if I'm feeling generous.
I've also found alien eggs (on Santa list in 2004), a rubiks cube (Santa list 2007), guitar picks and strings, marbles (Santa list 2002), a graduate build-a-bear (dd), earphones and power banks (not expensive, and constantly breaking/getting lost), naice shampoo, three colour pens, refill pads (hopefully they are studying), chocolate, spices and random Irish foodstuffs (for the one in Australia), etc, etc.
Basically I'm satisfying my need to shop for them (in pound shops and supermarkets) while spending very little 