I have used the Park & Fly rate at the Sea-Tac Hilton. I recall that many Sea-Tac Hilton properties offer such an arrangement -- some may be seasonal. If available a Park & Fly rate will appear with any rate inquiry on the Hilton web site.
I would compare the cost of the hotel night with other parking options. It may make more sense to stay at a lower rate at the HI and park at an off site lot like Shuttle Park 2 -- $7.50/night AAA rate. Most of them are right along (or just off) International Blvd. where most of the hotels are.
One other consideration is that, since you may be travelling around a "Spring Break" period, having a confirmed spot (e.g via hotel park & fly) may make sense. Last year, the SB saw a lot of parking lots full.