I am going to be helping a kid at school with a goat playground type deal. I plan on using treated lumber where this playground touches the ground. But since this is done for the school we do have a budget, and doing the whole thing out of treated wood is not preferred, but maybe we can. I think by using regular wood it will quickly go bad, with the time were spending I want this to last. What can I put over the wood to prolong the life of the project? The only thing I thought of was the stuff used for coating a deck.
Use a naturally weather-resistant wood like cedar. It is relatively cheap and easy to maintain. A lot of playgrounds, fences, decks, and outdoor furniture are made out of cedar for this reason.
Cedar weathers to a silver color, becoming more gray with time. Or you could stain and seal it, just like a deck, if you don't want the gray color.
There are other naturally weather-resistant woods out there (e.g., teak), but cedar is probably the cheapest and most widely-available.