If you can get a driver for the evening. Do not exert yourself for any reason. Plan in advance to have her picked up right from work or the house. Have nice things waiting in the care for her, a card (personally made not bought), flowers, what ever else you know that she likes.
They have these nice cruises that are really nice to take the lady on as part of a romantic evening. The is about two hours so you have two hours to make her day brighter. Have poetry ready for her if you are not an on the spot poet, memorize it in advance. Have the waiter have a small gift for her served with the meal as dessert.
Have the car pick you two up, drive around for a while, taking her some where that is special to you two, just so you could look at the stars together and hold one another. Women really take pleasure from being help and feeling secure in thier guy arms. Oh yeah, don't forget that dramatic part where you look deep in her eyes and tell her how beautiful she is...
The real key is making sure that she know that you are luck to her her in your life. Once she feels that way you are all set.
Best wishes always,