The munchkin and I did the Hiawatha trail yesterday and it was simply fantastic. Some tips:
If you can go on a weekday, it’s 1/3 as many people.
Bring a GOOD headlamp (or bike lamp), extra batteries, and if you wanna be a good scout an extra headlamp just in case.
They have several places along the trail with coolers of water, so a single water bottle will probably do you fine.
Drive to exit 5 in Montana, follow the Hiawatha signs to the East Portal trailhead. Get your $9 trail ticket.
Bike downhill, plan on 2.5 hours (we did it in 2 but we liked to go a bit faster) to get to the bottom (“Pearson” portal). Take the $9/person shuttle back up to the west end of the tunnel (“Roland” portal) then it;s about a 10 minute bike right through the tunnel back to the east portal where you parked.
Remember the time zone difference! The start of the trail is in Montana (MDT) and the end of the trail where the buses are is Idaho (PDT). The bus schedules are PDT.