Being an avid steam guy, I tend to lean that way first, but between 1994-1997 we did an "America By Rail" series with Amtrak trips, and added side-trips on tourist train rides. So I have a pretty good feel for whats out there.
US Steam Must-Do's
Cumbres and Toltec
Durango and Silverton
US Steam close seconds
Heber Valley...Diverse scenery in Utah
Nevada Northern(although I haven't been)
White Pass and Yukon(when they run a long steam trip)
Cass Scenic Railroad (West Virginia Shays and Mountains)
Great Smoky Mountain Railroad (same folks as D&S)
US Diesel Trips
White Pass and Yukon(If the steam doesn't run)
Verde Canyon(Arizona). This is a sleeper trip,
and one that most folks miss entirely.
California Western, California Remote forests
Mt. Hood Railroad. Switch-back, and mountain peaks
Foreign
(I have to qualify this cause my trips to Europe
were before steam was completely gone).
Switzerland(lots of cool trips there from what I have seen, if scenery is your thing)
Harzgebahn(German Narrow Gauge system in the Harz Mountains of Former East Germany)... A Must!!!
Wolstyn, Poland...Want to run a steam train...go to Wolstyn. Several engines still run on regular trains here. Not well known.
Don't forget about the UK, as they have plenty of steam running also.
Greg Scholl