Ooohh, pretty lights......It's one of the many things I love about Vegas.
And the observation deck on the half-pint Eiffel Tower allows a lovely vantage point to oooh and ahhh over all the neon (and the Bellagio fountain shows, to boot), so I would suggest going at night. It's open until midnight so you should have plenty of time to fit it in among the general debauchery/gambling losses.
It's $10 for the trip during the day and $12 at night*, essentially the same as just one hand of blackjack at one of the cheap tables, so I don't want to hear that it's too expensive. We were lucky that it wasn't too cold or too windy, especially since a line tends to form to head back down, but they seemed to pack enough people in the elevator so it moved fairly quickly.
Take your lady up here, she'll love it.
*A tip for the frugal: You can find a two-dollar off coupon in the Vegas Values mini-mags.