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The easiest and cheapest way is probably to use a few drops of bleach. Don't use any bleach that's scented, or has any other fabric-softening additives, as those chemicals will almost certainly make you sick. However, the pure, unadulterated stuff is commonly used to treat water in 3rd world countries or areas where the water supply is not proven to be potable. I've seen a number of backpackers who use bleach to treat their water, although it's not a very popular method. That said, I am not aware of much of a consensus among backpackers or anybody indicating that bleach is unsafe when used in small quantities to treat water. It can add a nasty chlorine odor to the water, which some people don't like, but since you only need 7-8 drops per gallon, the other chemicals (yes, there are a wide variety of chemicals besides the chlorides which sterilize water, even in unscented bleach) are supposedly safe to drink in these concentrations.
If you aren't comfortable using bleach, the absolute easiest way to treat water is to just scoop it up with your bottle from a stream, prefilter it if you don't like the taste of tadpoles, and add a chemical tablet. Everyone knows about iodine. It's easy to use, but adds a nasty taste to water. Chlorine dioxide is an odorless and tasteless alternative. It comes in tablets made by Potable Aqua and Aqua Mira, and although the tablets are about 4 times more expensive than the liquid treatments (the drop version of Aqua Mira), the tablets are a bit easier and quicker to use (also more effective than the liquid version), so I think they fit your criteria the best.
Boiling and filtering, of course, are viable options, but nowhere near as easy to use as tablets.
