Liz Hendrickson and I tried this app based Scavenger Hunt yesterday and really enjoyed it.
Liz has lived in Alexandria for many years and she said she enjoyed it and learned some new things. The app asks you if you are a resident or a visitor to the area.
We walked at a leisurely pace and did the scavenger hunt in about an hour and a half. We didn't take any breaks and we did not do any of the in between challenges.
If you decide to do this... I would recommend splitting up into smaller groups (e.g. 4 teams of 6 or 7 people). Each group will need a cellphone with the Let's Roam app. There is a 7 day free trial. The monthly cost for one device is $19.99 and covers up to 6 people. Make sure you charge your cellphone.
Here are some screenshots to give you an idea of how it works.