I am doing an outside project using some lumber that is not pressure treated. Some of it will be exposed to the weather. What would be the best way to protect it? Should I paint it or or use a water repellent like you would use on a deck?

A good all weather solid colour type stain will do.
Re do as required every few years.

Depends on what you are using it for. A mix of pine tar, linseed oil, and turpentine works fairly well. Burning it works good. Depending on the wood, perhaps nothing. I have a set of white oak step 25 years old, never done a thing with them. They set directly on the ground. If you have an old fashioned hardware store locally go in and ask them. The little stores seem to have a lot of knowledge available. I know that there are some products available that are supposed to be real good, but I don't know anything about them.

If its roughsawn you dont have to do anything planed needs to be treated with something regularly

It would depend on what type of wood. Case in point......if you cut a Red Oak down and leave it right where it fell, it will be punky and rotted in a matter of a very few years. If you cut that same tree, then put it up off the ground where it will be dry most of the time, it will last decades. Once back in the early 90's my All Purpose Boy and I were Turkey hunting with a friend in central Florida. The gobblers we were hoping to tag lived mostly in a damp section of bottomland that was about half Bald Cypress trees. Mixed in amongst the big cypress were the occasional stumps from cypress that had been logged out just over 100 years before. The size of some of these stumps were impressive, so impressive that I took my son's photo standing next to one whopper. Bottomland cypress tends to butress heavy just above the ground. The loggers had to cut notches in the trees near the tops of these butresses so they could jab in a plank to stand on as they sawed the trees down. There is video of guys doing this very thing back in the 20's and 30's. The notches they cut out laid right on the ground where they fell a century earlier.....the very damp ground. These chunks of wood are probably still there today. Old Growth cypress is extremely rot resistant stuff.

Depending on what your using for and can swing the cost get some western red and you won't have to do a thing with it.

This, however, how many people remember to "re do as required every few years"? Not many

Also, there are many other thing that come into play; species of wood, heartwood or sap wood, Climate and environment of where the wood is being used.
Does any wood rot easily in Kansas? I picture it as a pretty dry climate, but I wouldn't know.

Here I worry more about carpenter bees than rot.

I'm building a timber framed structure myself out of doug fir. All the recommendations I got was to use linseed oil mixed with mineral spirits. It's best to do it on a warm dry day , the linseed oil penetrates into the wood.

The boards are 2x6's used for a base for a hunting platform. The boards covering them are treated. I may get some stain like used on decks and do it all.

Cunap is what you need for that. It's copper napthalate and will turn the wood green buts its basically the same stuff used for pressure treat. A good soak or slathered on until the wood can take no more is how you apply.
Rustoleum also has a product called creocoat in clear and black that carries an in ground warranty. I image it is also copper based as the creosote of old is now "bad for you".

Paint or waterproof sealer should help.

Treat first with Borax.

I used zinc borate to protect the logs in my cabin. It works as a fungicide and an insecticide...it keeps bad stuff from attacking the wood. After the borate treatment I used Q8 Outlast log oil as a preservative. Water beads up and rolls off the wood. Everything seems to be holding up well.

I'm confused here. Why use untreated lumber between the ground and treated lumber ??

I see White has asked the proper question...lol

Lowes has a roll on called cooper it is high.

Deck stain. Paint kids playground equipment with that stuff from lowes and it looks brand new. 6 years old.

I used untreated 2x6s because I had access to free ones in like new condition. I bought a gallon of waterproofing deck stain today. I think it will do what I want. Thanks for all of the advice.

Burning the outside with a weed burner does work very well. Brush the loose char off and then use the water proofer of your choosing.