Remember that the continental US (covered by most airlines' 25k-mile awards) includes Alaska, so a weekend around Anchorage is a great way to get the most bang for your mile. There's a Sheraton in the city and incredible scenery all around (I'd highly recommend a drive down the Kenai Peninsula or a quick trip into Denali Park). Anchorage itself is... well, quirky. Not entirely in a good way. But it's a decent base for excursions into the surrounding area.

Summer has the best weather and absurd amounts of daylight, but award flights may be scarce. Early to mid-autumn is still beautiful though.

If you'd prefer something a bit closer, Portland and Seattle are great cities in their own right and also surrounded by natural beauty. Also best to go summer/early fall, unless you like relentless drizzle.