National Parks Traveler suggests this is a great time to go looking for wildlife, and the time’s coming soon to see peak fall colors near the high peaks:

Timing fall’s best colors in the Rockies is tricky, as they’re at their peak for such a short period, usually no longer than a week or so. Sometimes that peak arrives at the end of September, other times at mid-month. This year I’m guessing that the colors will peak in mid-September because fall seems to be descending more quickly than in recent years.
June and July were hotter than usual this summer here in the Rockies, while August was cooler. Too, a few weeks ago I was up in central Idaho and awoke a few days to frost on our car. Perhaps those are not the most scientific reasons for my gut feeling, but I’d wager fall colors in the northern Rockies will be at their best at mid-month.

I’m guessing much the same’s true for the Sierra.