My advice, assuming 35,000 points, would be a 2-zone and a 1-zone trip, since you can use the "break" between the two awards to book a far longer trip than you would be able to get if you were booking it as a single award. One that I've heard suggested would be Wolf Point-El Paso (which would run via PDX and LAX) and then El Paso-Miami (via CHI and WAS). I think that would get you 7 nights on the train. The reverse could be routed through NYP instead of WAS, which would get you an additional couple of hours on a train (WAS-NYP plus about an extra 90 minutes NYP-CHI vs. WAS-CHI)...or possibly over the Cardinal, which would probably net the most time WAS-CHI.
Another fun two-zone option would be El Paso-Chicago...via LAX/PDX. If you're up for a short trip to El Paso, SEA-LAX-ELP and ELP-LAX-PDX-CHI would be allowable, and you'd get four nights on a pair of very nice trains in the deal (as well as two nights on the Sunset Limited). Extending the start of the trip to Wolf Point would add in a seventh night overall...but it would leave you with a pair of questionable endpoints. It all comes down to how far you're willing to go to stretch your points. My advice in either case would be to use cash to get to/from Wolf Point (it's probably going to be $350-400 or so in a sleeper; I'd quote an exact number, but Amsnag is acting funny for me) and roll from there.
One thing I'd say is that 35,000 is an awkward point to be at because it's not enough to do a two-zone round trip (such as ELP-MIA and back), so you're likely to be caught ending somewhere you didn't start at unless you either start on a zone boundary and do a loop or you "eat" a reservation that doesn't take full advantage of one of the zones.
If you wanted to start or end in Seattle, my advice would be one of the Seattle-Miami options (either way) or to shoot for something wacky like SEA-ATL (via CHI and NOL) and then ATL-DEN (again, via NOL and CHI) if it was allowed (NOL is a bit funky, as you'll see in a moment). You'd have to spring for a hotel in NOL, but that would get you 2-3 nights SEA-CHI (2 via the Builder, 3 via the Zephyr/Starlight), 1 CHI-NOL, 0 NOL-ATL (day train segment, but you'd have a room and be on board for most of the day), 0 ATL-NOL (see NOL-ATL), 1 CHI-NOL, and 1 CHI-DEN...it would be 6 nights, but you'd have two full days on the Crescent.
