I spend a week in Pella, IA every summer. I enjoy wandering the square and going into all the shops. If you stumble upon Van Veen chocolates, be sure to buy some (the chocolate covered caramels are delicious!). They are homemade and unlike anything I've had before. Speaking of food, you can not take a trip to Pella without having a Dutch letter which is an "S" shaped flaky pastry with an almond filling. I prefer the ones from Jaarmsa Bakery. They sell lots of other treats as well, so they make a nice afternoon stop.
Right off the square is the Klokkenspel (free!) which is a clock that has a show telling this history of Pella. Check the times and make sure you cross the street to see the backside of the show. As for tours, I really enjoyed the Historical Village and Vermeer Windmill tour. It was very reasonably priced and seeing the working windmill was really unique. It is located less than a block from the klokkenspel. Another tour I have taken is the Scholte House. It was ok, but the highlight for me was the garden which is free to wander through. Another unique garden to see is the Sunken Garden. I have walked there from the main square but it was a bit of a walk in the summer. It is such a pretty spot in town and would be a nice spot to picnic.
As for food, the Windmill Cafe has breakfast and lunch and is good and reasonably priced (I don't believe they take credit cards so keep that in mind). Smokey Row Coffee is another good spot to grab lunch. I like getting meats and cheeses from the meat markets (Ulrich or In't Veld) and making a meal out of that. If you will be grilling at your campsite, this would be a great spot to pick up your meats. Nearly everything is made on site and delicious.
While I haven't been there, I hear the Aquatic Center in Pella is really nice. I have talked to lots of people who take their kids there for the day and they have positive reviews. Pella is a cute, quaint town and it is unique to see all the buildings in town that must adhere to the Dutch guidelines. I'm pretty sure you will not see another Wal-mart facade that looks Dutch!