Yeah, tangerines and oranges are great. If you life in an area with a Chinese supermarket, check it out when New Year's rolls around. It's Feb. 18th this year, right? An Asian market should have plenty of goodies that you can buy. Potted chrysanthemums and paperwhites are always a good idea. However, watch out for the chrysanthemums. My family (we're descended from the southern Cantonese-speaking part of China) always had mums and paperwhites to mark the New Year. However, I brought a pot to my friend's Taiwanese parents and they're like, "Isn't this Japanese?" So... there are regional differences.
Sweets/snacks are always good. There are seeds that you crack open to eat the meat that you can buy bags of. There's a variety of Chinese candies, like a creamy, strawberry-flavored hard candy (red and gold shiny wrapper... can't miss it), chewy coconut strips, and those rice candies where you can eat the inner wrappers (I always found that kind of gross).
The red envelopes are for children and unmarried adults only. They're actually not a bad idea if you can't find anything else, but I wouldn't gift married people with them. I know handing out money in the workplace is a little odd to Americans, but a five in a red envelope is pretty symbolic and I think your co-workers would appreciate that you took the time to acknowledge the New Year.
Another thing to do other than gifts would be to take everyone out for dimsum. I'd avoid an actual gift (like an item in a box tied with ribbon) as it's not very traditional to give out boxed/wrapped gifts (as you would for a birthday) for Chinese New Year.
The big, broad, generalized rule of thumb with Chinese: you can't EVER go wrong with food or money as a gift. Keep it simple and you won't stray on any superstitions. Oh, actually, avoid giving things in groups of four.
That means death. And don't give scissors or any other object that "cuts" off the flow of good fortune to the New Year. Avoid things that are purely white (mourning). There's more, but I'll stop here. ETA: Depending on your co-workers and how traditional they are, if you give them something, they will very likely reciprocate. Just FYI, in case it's something that might catch you off-guard.