Make a lot of jokes about the routine things, particularly the common annoyances--Also, mention the long hours--you could say something about the fact that health care professionals(whatever her specialty is) don't need hobbies, five hours of sleep is enough--You could say that after she'd been there twenty five years (or however long she's been there) of running through the lobby or waiting room or whatever, she sat down one day, and decided that she liked it--you could say that she's leaving because she wanted to find out what her house looked like in daylight--

Tell a few stories about how she did things, share some memories of difficult situations that she had to deal with-- sacrifices that she made--little things that she did for people that people may not have been aware of--

Finish up with something about her having been an excellent whatever she was, but most of all, she was a great friend--

Wish her the best, welcome her to the world of retirement, and tell here that she'll be so busy she'll wonder how she ever managed to find time to work.

Keep it short, throw in a few jokes, and everyone will love it.