My daughter broke her humerus last summer, right after she turned 3 by (get this...) rolling off my bed in her sleep. :( She also broke her clavicle during birth, but luckily all we had to do for that was pin her arm under her shirt and wait a few weeks before starting tummy time. (Can't really cast a newborn!)
The hardest part is bathing. I found that the easiest thing to do was to cut the toe off a large sock, and roll it into a doughnut. I would roll it over the cast, so that it sat over the edge of it (to absorb any moisture that seeped in) then I would put a bag over the whole thing, and use a hair tie to hold it in place. Produce/meat bags from the grocery store work great. :) I also used a few hair ties along the cast to keep it closer, so it wasn't getting in he way too much. Instead of giving her a bath, I used our detachable shower head and tried to avoid the area as much as possible. I might put a little water in the tub, but only a few inches. If there was more than that, she would want to play.
we didn't really have to do anything different activity wise. The cast provides a lot of protection, and she will forget she even has it on before too long. :) I was a nervous wreck about letting her run or play on toys at the park... But she was perfectly fine. (I did turn into a helicopter mom for a while there though, and went with her the first few times... And I didn't let her run full-out much either... Lol.)
For feeding, I just wrapped a dish towel around it and secured with hair ties. She will have to either switch hands for a while, (finger foods will make things easier there...) or allow you to feed her. You could also try finding a pair of those cheap stretchy gloves, and cut the fingers off so it can slide over the cast and protect it, so she can still use that hand a bit. My daughter got a kick out of eating soup through a straw. :)
For clothing... Good thing it's summer! Tank tops!
Also, get some moleskin! The part that goes between the thumb and he rest of the fingers, and over the palm gets gross, fast. The padding also tends to wear out quickly too... So use the moleskin, and stick it on and tuck the ends in a little bit. It is stickier and more comfortable than a band-aid, and will protect her hand from chafing. Plus, when it gets gross you can peel it off and replace it. :)