I'm a big fan of Alaska miles, because of the free stopovers on one way awards and great mix of partners from multiple alliances.
Alaska Mileage Plan Basics
Before we get into potential redemptions and stopover destinations, let's go over some basics about the program.

Partners

Alaska has a varied array of oneworld, SkyTeam, and non-allied airline partners:

Here are the award charts for flying Alaska and its partners.

LATAM and Cathay Pacific award space cannot be found on alaskaair.com. Search with British Airways award search engine instead.

lf you aren't already a member of Alaska MileagePlan, Alaska Airline's frequent flyer program, register before April 30, 2017, and take a paid Alaska Airlines or Virgin America flight by September 30, 2017 and you'll earn a bonus of 5,000 Alaska miles.
How to Get 5k Bonus Miles
If you don't have an existing MileagePlan account...

Register here before April 30, 2017.

I tend to toot Alaska MileagePlan's horn quite a bit because I myself am a big fan of Alaska miles. You get free stopovers on one way awards, Alaska has a great mix of partners from multiple alliances, and their award charts are filled with sweetspots. Their miles also aren't the hardest to come by, since Bank of America issues a personal and business co-branded Alaska card and SPG points transfer to Alaska.

One of the perks of the Alaska Airlines credit card (both the personal and business version come with one) is an annual Companion Certificate.

A few weeks ago Scott posted about The Cheapest Ways to Get to Hawaii.

I recently came across another sweetspot to Hawaii I wasn't aware of. It costs just 17,500 Alaska miles to fly Alaska Airlines between the mainland United States and Hawaii. And for a little more (20k one way) you can fly Virgin America from a couple west coast cities to a couple different Hawaiian islands.

And since we're talking Alaska miles, you can book one free stopover per one way.

The majority of us earn heaps of miles through sign up bonuses, maximizing category bonuses on credit cards, and flying. And at the end of the day, credit cards are the bread and butter that enable most of us to fly free in First Class.

But there are lots of other ways to earn miles, many of which are passive and all of which require little effort.

Update 1/18/17: A reader reports that he tried to book San Francisco to Boston Virgin America First Class award space with Alaska miles--he tried various summer dates--and every time received an error message. When he called Alaska, a representative confirmed that there was no award space on the days he checked. Who knows if that will happen to you, but beware that the Virgin Atlantic First Class award space displaying on alaskair.com could be phantom. 

Alaska Airlines and Virgin America merged earlier this month, and it's now possible to redeem Alaska miles on Virgin America flights.

UPDATE 1/10/17: If you have a Virgin America Elevate account and an Alaska Mileage Plan account that were both opened prior to December 5, 2016, you should have gotten/be getting the 10k Alaska mile bonus soon. Alaska shared the following statement with The Points Guy:
"We understand that many Mileage Plan members have questions about a surprise 10,000 mile gift that started appearing in some accounts last night, and we wanted to help put some of the mystery to rest.

Starwood Preferred Guest and Virgin America Elevate are ending their partnership on January 6, 2017, making that the last day that you can transfer SPG points to Virgin America Points.
Why does this matter?
In April of this year, Starwood Preferred Guest added Virgin America Elevate as a 1:1 transfer partner.

Virgin America and Alaska Airlines have merged.

Starting January 9, 2017, you can link your Alaska Mileage Plan and Virgin America Elevate accounts. Then you can transfer 1 Virgin America points to 1.3 Alaska miles.

That is almost exactly the fair ratio in my opinion.

For a 90 trial period, Virgin America will match the status you have with either JetBlue, United, Delta, Southwest, or American Airlines to either Elevate Gold or Silver. The offer is valid until 11.59 PM PT September 30, 2016.