Check multiple libraries - we're in Virginia but there are three libraries within a 10-15 minute drive, all with story times on different days! When my girls were that age they went out every day and library story hour was heavy in the rotation. Our library also had Babygarten classes - you had to register for the classes in advance but they were free (registration was to control the number of kids there so it matched the number of musical instruments they played - and it's geared for kids that age). There was also a preschool music class at the library but the timing never worked right for that one.
Check to see if your county Community Center has a Tot Drop In - ours had one 2-3 days a week, only during the school year, that would put mats on the floor and tricycles/bikes/slides/whatever they could find in the gym and let kids run around for an hour or so. It was $2.50 per adult, kids were free. We got our au pair a 10 visit pass every few months.
We encouraged our au pairs to go to McDonalds and Chick Fil A play places - I know not everyone likes them. If some near you are appropriate for your kids' ages, empower your au pair to take them! We also visit mall play places.
Check out if MyGym or Little Gym (or whatever is near you) has drop in times, or even if there's a class your au pair can take both kids to.
We put our kids in a church preschool (2x a week at 2, 3x a week at 3, three hours a day) when they were two - maybe something else to look into.
Finally, consider whether there is a cost associated with any of the activities and make sure you reimburse your au pair for any money she is out of pocket (which includes buying her coffee at McDonalds as "admission" to the play place). Happy au pair = happy kids = happy mom.