Hi Auntie Katie,
I did nanny duty for a couple years with the same age - two girls. My best advice is get on their level (literally, sit down and often you'll find yourself shuffling around on your knees!) and just follow their lead.
Some favorites of my nieces:
Balloons - helium filled or not. (Follow appropriate safety measures, of course.)
Musical toys, like maracas, harmonica, or bongos. Play along with your finely-tuned children's Pandora station! Music is a huge helper - we usually had it on all the time.
Dance! Scoop up that little one and boogie together!
Builders and Bulldozers. You set up a few blocks (or mini-cones if you have them) and as baby goes around knocking them over, you put them back up. This can last for HOURS. Don't forget to mix it up and switch roles if you can.
Go outside for a walk. Use a stroller and as you walk through the neighborhood, talk to the little one about what he's seeing.
Follow your instincts and try to look at things from the little one's perspective. This will keep you on your toes and your ideas will flow.
Good luck and have fun!