I think asking her out on a date would be a good idea and then taking her to somewhere really nice not in an expensive sense but e.g. to place which does food or particular dish she likes and then taking a walk or going onto a nice place for a drink, cocktails are always good. Do something from the heart, don't be cheesy!!
Asking her out on a date I think will say to her I like you and I want to spend time together as more than friends. If she says no then you tried but make it clear you still want to be friends.
I hope this is useful!
Root xx