First, the OP didn't say "once every couple of years" -- rather, it was international every couple years and skiing (presumably domestic) once per year.

Second, the OP ALREADY has 17,500 miles.

Third, your claim that "it's unlikely that you'll even be able to keep your miles past the 18 month expiration window" is just dead wrong. There are DOZENS of ways to extend the expiration date by 18 months (from current date), simply by earning OR redeeming miles. Most of these earning and redemption methods don't even involve flying -- i.e., hotel, car rental, merchandise, dining, charity, and numerous other activities. (Resetting the expiration date could be as simple and instantaneous as donating 1,000 miles to charity on the United website, just as one example.) All this has been discussed hundreds of times on this forum, as any seasoned FTer should realize!

As for the OP's original question, I think the "best" way to redeem miles is the one that suits one's own interests and lifestyle. If I were the OP, I would study up on united.com (under "Mileage Plus", click on "Redeem Miles"), as well as look through the FAQ sticky here in this forum for further info. It's certainly possible to keep one's miles (since it's easy to prevent them from expiring) and accumulate more if one's goals are to redeem awards that require a higher balance than currently owned.