When you travel between Coeur d'Alene and Portland, I'd recommend taking 90 west to Ellensburg, 82 south to Yakima, Wa St 12, the Wa St 410 to Mt Rainier National park. This is THE most spectacular way to approach Mt Rainier (and it's on the way
I'm not sure if there are vehicle restrictions on size (you can ask 1-877-617-9950), but I think the single most spectacular place that you can drive to in the park is Sunrise. You will begin to wonder if you aren't going to the top in your bounder
You'll see some CCC work that was done in the park on this road, if you stop on the last hairpin before the top (only large parking area before top), and usually if you arrive before 10 AM you will have no trouble finding parking at the top (don't laugh, it happens.)
If you plan to overnight in the park, as I'm assuming you don't want to drive all day - make reservations here, SOON ! (877) 444-6777 - stay at Ohanapeckosh (I'd say White river, but I don't think they can handle a bounder - Make sure you ask about vehicle length for both - there are some out of the park places that can handle the length outside the SE entrance see http://www.nps.gov/mora/planyourvisit/uplo...campgrounds.pdf
The following day, head south out of the Park, pick up WA St route 12 towards Packwood/ Randle. At Randle take 131 south. http://www.fs.usda.gov/Internet/FSE_MEDIA/...prdb5189063.jpg
Take State road 99 to Windy Ridge to overlook Spirit Lake and look down the throat of Mt Saint Helens. If your vehicle were smaller, I'd say take route 26 for a more spectacular approach, but it is so small that it almost turns into a one way at points. In days past, tens of thousands of huge trees (four feet in diameter) were blown down by the blast when MSH blew her top. The pyroplastic wind swirled around the near by mountain tops, making 'hair' like patterns out of these timber.
Before it was logged out, you could drive thru here and get the distinct feeling your vehicle was a flea on a dog's back and the trees were the hairs.
If you have the time, the 'Ape Caves' are WELL worth doing (see above map). You can rent a Coleman lantern at the location (but make sure you take at least two flashlights too, as without light - the dark is absolute) and you can walk one of two ways into a long since cold 'lava tube' cave (something your boys won't soon forget.) The way that goes down the stairs never gets very narrow. If you could get a 4X4 down there you could drive to the end (about a half mile). It has no exit other than the way you came in.
The other way is not for the claustrophobic. Ok, it's not all that bad, I've done it twice, and while you do have to squeeze thru one spot, and climb over a cave-in that caused a huge amount of rubble in a domed area so that you have four feet of clearance, it is very manageable. You will see re-melt, lava that cooled to rock, then melted again from the lava flowing down the tube. It is frozen in time as it dripped from the ceiling. And you'll have the opportunity to climb one small frozen lava fall in the middle (6 feet tall and very doable) which is sure to stick in the boy's minds as a highlight. It is pretty much impossible to get lost, as the very few arms in the cave rarely last longer than 20 feet. The north route ends with a climb on a short ladder to daylight. I can promise that if you do the north side, you will be the 'coolest dad' for the rest of the trip.
At 50, I STILL enjoyed it.
Don't worry about cave-ins.....I'm told the last one happened thousands of years ago, and if it didn't happen with this last eruption, it AIN'T gonna happen in our lifetime.
After Mt St Helens, you are a hop, skip, and a jump from I5 and Portland.