Buy her a day at a health/beauty spa for a facial etc. and comment on how beautiful she looks when she arrives home after the spa. It has never failed.
That's a great idea.
Get a basket... in it, put:
the spa certificate (you can make it a specific dollar amount... with no appt. date attached)
couple of candles/candlesticks
a good book of her interest
a small box of chocolates
a comfy pair of socks
a set of comfy pajamas (not lingerie ~ it's for her... not you)
if you want to add in some jewelery, that's always a winner.
p.s. don't worry about waiting until the last day. She gets the gift on her birthday whether you bought it two weeks in advance or two hours in advance. Just don't tell her you waited until the last day, and you're still golden.