I've used 2+ million miles for all the usual things (upgrade, trips without the Saturday stayover, etc.), but here are four of my most memorable:

I've taken each niece/nephew to DC for a long weekend as they reach 13 or 14 - old enough I can enjoy being with them for several days in a row, and they can appreciate the museums and memorials. I call it "Uncle John's Civics Lesson."

Flew one sister from Spokane to Houston for another sister's 2nd wedding - she thought work would tell her she was too busy, but when she was blue the day before the wedding they said "WHAT DO YOU MEAN, your missing your sister's wedding. Of course you can go!" The airlines wanted $1,200+

Flew myself to Fargo, North Dakota (seriously!) since that would complete I've been to all 50 states.

And (the most morable use) flew parents, myself and five siblings, with spouses, to Hawaii for a family reunion. No kids (who really wants to watch a 5-year-old on the beach), but I told them all that United's Hawaii freebies allow a stopover, and it didn't have to be on the routing (at least that was OK in '92). Houston went home by way of Spokane to see that sister's house & kids; Spokane went by way of Charleston, SC (really!) because she considered this a "found" trip and she'd always wanted to see there. Denver visited friends in San Diego on the way, and parents (also Denver; I gave them first class as an anniversary "bonus") went home by way of Florida.

For memories, maximum value, and everything else, that was probably my best use. But guess what: I live in San Francisco, so fares to Hawaii are cheap enough that my partner and I PAID for our tickets (and earned more miles)!