I've seen posts where you can get a platic covering for casts so you can still go swimming. I don't know where to get them, but if your niece wants to swim, she might be able to with these covers.
Since she has a broken ankle, depending on how much weight she is allowed to have , or not have, swimming might not be a good idea, especially if she is in a lot of pain, or if it hurts if someone bumps her.
I'd call the Orthopedic office, or a PT or rehab place if it looks like something for her to try.
http://www.waterproof-cast-cover.com/ is one i saw online.
I'd recommend going with an offsite wheel chair rental. Maybe you'll be able to get a child size chair. An appropreiate size chair will make her day more comfortable, as the arm rests of an adult are probably to high.
Also, if she needs to keep the leg elavated. Walker has the wheelchair legs, that can raise. Last time i needed one, it was an additional $10 charge per side that needed to be raised. It was a flat fee on top of the regular rental charge. You need to note that at time of reservation, they only put the raisable legs on when requested.
Connie