What a nifty idea! if you want creative envelopes, go to Maps, map the route from your letter to the other person's mailbox. Print them up, fold them into 8 by 11 envelopes (or smaller if you are sending cards). This would be cool for pen pals
An alternative to chlorine pools, the BioTop Natural Pools use plants to keep water clean and clear. Wikipedia has some more info on natural swimming pools as well. These could make a great addition to a permaculture design site.