So we landed on semi-transparent concrete stain. It turned out well; and it can be accented with outdoor rugs or even a patch of fake grass (which is what we did). The total cost was $300 for about 500 sqft of coverage, including tool rental and purchase - that's $0.60 a foot (way less than the $5 a foot tiles).
Unfortunately we didn't take too many pictures of the process, but we did learn a couple of tricks that should be useful for people without concrete staining experience. Hit the jump below for more details.
--Power washer ($30 for 4 hours, we rented ours from Home Depot)
--Extendable rollers ($13 ea) and roller covers ($8 for 6), or a sprayer (we used rollers)
--Paint trays (about $1 ea)
--Concrete stain (about $25/gallon; budget about one gallon per 75 sq ft)
--crappy clothes and shoes to wear during the reno
STEP 1: Move everything and power wash
The biggest pain in this whole project is clearing off the space. We moved everything into our driveway for a day and a half. Hopefully you actually have a yard where you can move stuff without too much trouble.
Also, make sure you power wash hard enough to get the dirt and moss off. This will take longer (it took us more than three hours of solid washing to clean the whole slab), but it makes a huge difference and will help the stain stick.
STEP 2: Realize you're going to need a ton of stain.
This picture was taken after one coat. You can clearly see the roller marks, and it's not pretty. We wound up needing THREE TIMES as much stain as we'd thought. The cans say they cover 350-500 square feet, but to cover properly, expect to cover 75 sqft per gallon.
Note: your mileage may vary if you feel confident using a sprayer, as sprayers seem to be able to cover more evenly with less stain. But if you're not that experienced, like we aren't, normal rollers/brushes work fine.
I'd recommend buying an extra gallon of stain, too. I made 5 different trips to Home Depot the day we did this.
STEP 3: Stain until you can't stain anymore.
To make the best use of your power washing, and to preserve your sanity, try to get this whole project done in one day.
Pick a hot day to work, too. Direct sunlight dried each coat for us in about an hour, which means you can start the next one as soon as you finish the previous.
Using rollers and normal painting techniques, this whole project took us about eight hours.