Thanks for the suggestions, everyone. We ended up doing a
ride in Snohomish from the Seattle Bicycle Touring Club's site, which was excellent--some rolling hills, but nothing too crazy, which was just as well as we've not quite mastered standing yet. But, we did cruise along comfortably in the low-mid 20s on the flats, which the stoker thought was great fun, so all in all, it was a success.
Daniel