Walking around the park (all the way) can be fun, great views from certain vantage points (like the ramps looking up and down the Anacostia) and you can stop at the highlight spots (like finding Tomterp on the emblem listing of all the lifetime season ticket holders 
) Good exercise for energetic little fellows.
If you get seats that allow access to the NS Club, there's a really neat large train-themed active scoreboard on one wall that the kids will love. the club is a nice break for the kids, especially on hot days. The trains on the scoreboard move to indicate outs. It sounds a loud whistle when there's a HR as well.
