Does her activity *have* to be restricted (to purposefully keep her off the leg), or can she go at her own pace? My 2.5-year-old just got a long-leg cast off a few days ago (she spent 4 weeks in it for a fractured tibia), and I was shocked by how quickly she was up-and-running with the cast on. When she first got it on, she wouldn't even try to walk, just sort of dragged herself around in an "army crawl." Within two or three days, she'd gone from that to crawling, to "cruising" along furniture, to walking. After a week or two, she was running, jumping, and doing everything she did before she got the cast on. Her cast wasn't a "walking cast," per se--but the doctor told me that, while "avoiding weight-bearing" was the typical protocol, 2-year-olds are (a) really hard to keep down and (b) pretty good about doing what they are able to do, so not to worry about it when she started moving around again. Anyway, unless it's a particularly bad break that requires that she absolutely stay off it, you might be surprised by how quickly she resumes normal activity.
In the event that she *can't*, though...my 2.5-year-old just discovered Playmobil sets. They have a million tiny pieces, so I wouldn't recommend them if your niece still has any propensity to put things in her mouth (or other orifices, I suppose--I watched my sister extract a small rock from my nieces *nose* last week), but my daughter will sit for literally more than an hour at a time playing with them. They also make something called "Playmobil 1-2-3," which lacks the tiny pieces and are safer for younger children (but the detail of the tiny pieces are part of what my daughter enjoys, I think).
Or how about a "sensory table" (by which I really just mean a large, shallow rubbermaid-type bin) which could be filled with water, sand, rice, beans, et cetera?
My kids love to glue things to paper or cardboard, such as dyed pasta, bark/small stones/grass/feathers/other things we find outside, rice...
Oh, another thing: I bought a big 24-or-so pack of permanent markers in every color, and let my daughter choose how she wanted me to decorate her cast for her (she chose Yo Gabba Gabba characters). I was shocked by how still and attentive she was for HOURS as I colored every last square inch of her cast in bright colors. We spent nearly a whole day doing it. But she also took a lot of pride in it and enjoyed showing it off to people. Right before she got the cast off, a doctor walked by her and said, "Wow, what a cool cast!" And when he walked away, she just beamed and said, "Mom, that doctor LOVES my cast!" She was really proud to take it home with her after it was removed, too--so maybe do something like that, if you can. It entertained us for a day, and it made her feel really special, I think. (Here
are links to photos of her finished cast!)