I've just returned from a couple of weeks in Southern Utah, and was there in October 2009 and October 2010 too, so only have fall and spring experiences of the area.
August would probably be mighty hot (it was already nearly 90°F in Zion last week, the hike out of the Subway was *hot*), so keeping to higher altitudes, as suggested by others above, is probably favourite.
The Boulder-Escalante Mail Trail is a great two-day backpack (16 miles minimum and three main drainages to cross, including the incredible Death Hollow, but watch out for poison ivy), but it will still get hot up on the slickrock during the day; there are quite a few miles of trudging across exposed rock and across wide tracts of sandy sagebrush, but at least water should be available in Sand Creek/DH and Mamie Creek as well.
You could also backpack down the 18 or so miles of Steep Creek Canyon (from Roundup Flat high on Boulder Mountain on Highway 12) down to the Gulch trailhead at the end of Long Canyon on the Burr Trail, but you'd would need to spot a second car or arrange a shuttle, (similar for the Boulder Mail Trail, though hitching is also a possibility.)
The river hike from Escalante eastwards to the bridge over Highway 12 (near Calf Creek) would be another suggestion. Willis Creek is fun and picturesque, as a fun easy end of the day hike.
Also staying high, the 9 mile Rigg's Spring Loop at Bryce cuts through lots of forest and might be OK in August, though the last 4 miles is an exposed relentless climb back up the Pink Cliffs. The 8 mile Fairyland Loop has a shaded mid-way stop at Tower Bridge, so is also a possible. I would probably favour day-hiking these two than backpacking two days Under the Rim.
In Zion, Angels Landing would require an early start, Hidden Canyon (and/or the stretch of the East Rim through Echo Canyon towards Observation point) would be hot but doable later in the day. The Subway is a bit of a must, start early but be prepared for a hot hike-out with sun in your eyes, with a stiff climb up the baking-hot cliff back to the trailhead at the end. The Narrows is a no-brainer for Zion in hot weather, flash floods permitting. Kanarraville Creek is also a good half day side trip, cool and certainly less crowded than anywhere around the Zion main canyon.