Fresh paint is the least expensive, especially if you DIY. AND, if you DIY, be sure you do a professional-looking job especially if you’ve chosen to use color. There’s hardly anything worse than seeing messy edges. Color is great if you’re not trying to sell, but if you are be sure to use a neutral color such as beige or off-white. You want your house to be appealing to as many as possible, and neutral colors will offend fewer people.
Again, neutral is best. All rooms should be the same. I’ve heard that it’s best when purchasing new carpet to upgrade the padding and not the carpet itself. Costs less but gives the same feeling of thickness. You’ll want to check this out with people who really know, though.
I’m sure you’ve heard of sellers giving an allowance for these upgrades and not doing it upfront. First, doing it upfront will cause your home to show much better; and it’s been my experience that most buyers want to have these items already done so they can move right in.