First the nuts and bolts: I’ll use a separate post for itinerary suggestions.
The weather: to tell if it is nice weather, look out the window. You can see Mt Rainier from Bellevue. If it is a rainy day, it will likely be the same there. If the forecast is morning clouds and afternoon clearing, then Sunrise will likely be clear all day and Paradise is likely clear in the morning with fog in the afternoon as it lifts off the lowlands. Sunrise is in the rain shadow so it can frequently be cloudy in town and it will clear somewhere after the small town of Greenwater on the NE side.
Silver Falls isn’t in the Sunrise area. It’s halfway between Paradise and Sunrise near the Grove of the Patriarchs which Bill recommended. I suspect you mean the Pacific Crest Trail which runs from Mexico to Canada and skirts along the eastern boundary of the park. A portion of it runs near Chinook Pass and is a lovely hike, but you may not have time; it’s a little out of the way. I’ll include more details in the next post for both of these.
The road is paved, narrow and windy and bumpy in a few spots where they have done some road work. It’s not a tricky driving situation, but whoever gets car sick should be driving. Otherwise, it’s just a normal road with speed limits varying from 20 mph in the congested areas to 40 mph elsewhere in the park and 55 or 60 outside. If you are not used to windy roads and are driving slowly, there are frequent spots to pull over to check a map or let somebody out for some fresh air. The road is 2 lane the whole way so it’s not like some of the California roads where it becomes 1 lane and if you meet somebody, you have to back up. For a California person, think Yosemite vs Ebbetts Pass.
The National Park Inn at Longmire has a couple of 2 BR rooms that would work for your family of 4. One of the BR has 2 twins and the other a queen with a bath in between. They are in the corner of the building. However, you will be very lucky to find a vacancy at either Longmire or Paradise. They typically fill up early and post Labor Day is a favorite of locals. There are also places in Ashford/Elbe outside the Nisqually entrance. Crystal Mtn Ski area on the NE side has lodging and is more likely to have availability.
Driving: The Bellevue Hilton is just off of 405 which is one of the worst traffic spots in the area. Leave early and come back late to avoid the slowdowns, but you will also be going against the main traffic flow so it won’t be too bad. If you will be leaving from the airport the next day, you might want to look at the Hilton at Seatac for the last night or two to make it easier.
Directions: From Bellevue, take I-405 south to Hwy-167 (the Valley Freeway) south.
Take Hwy 512 west to Puyallup (note that there is an exit just before this one that mentions Mt Rainier/Sunrise; you don’t want that one). After just a few miles on 512, as you are heading up a hill, take the exit to Hwy 161 which I think also mentions Paradise. You turn left at the light and follow a very congested road for quite a ways. This is your best spot for gas. There is no gas anywhere in the park and it will be more expensive closer to the park so this is a good spot.
Stay on 161 all the way to Eatonville (small town and watch your speed). At the 4 way stop, take a left (there’s a grocery store on the far corner). This is the Alder cutoff and saves a little time. At the next stop sign, turn left onto Hwy 7 along Alder Lake and follow it into the park at the Nisqually Entrance. This road continues through Longmire and ends up at Paradise. About 3/4 of the way up to Paradise, note the Stevens Canyon Road to your right. Once you leave Paradise, you can either drive back down the way you came and turn left onto Stevens Canyon, or, if the Paradise Valley road is open, take that one way road down to the stop sign and turn left onto the Stevens Canyon Road. The Stevens Canyon road connects the west side to the east side of the park. At Ohanapecosh, it meets up with Hwy 123 which goes along the east side of the park. Turn left and follow up to a stop sign at Cayuse Pass (which doesn’t seem like a pass). Turn left onto Hwy 410 (a right turn takes you to Chinook Pass and the Pacific Crest trail: Option 4). This stretch of road gets pretty bumpy from frost heaves.
As you head down the hill, watch for the left turn to Sunrise and stop at the White River entrance to show your receipt/park pass again. Follow this road all the way up to Sunrise at 6400’. Return to Hwy 410 and take a left turn all the way through Greenwater (ice cream or coffee at Wapiti Woolies or a burger and beer at Naches Tavern) and into Enumclaw. Enumclaw is your next best gas option. Watch your speed in Greenwater.
From Enumclaw, take a right onto Hwy 169 and follow through Black Diamond (watch your speed) and back to I-405 in Renton. Take 405 north back to Bellevue. Another option for dinner on the way home is Krains Korner just north of Enumclaw on Hwy 169; it’s a historic old restaurant but check their hours first.
Side note: the route easyrider suggested is from the perspective of somebody who lives in Yakima, which is east. It is not the fastest option from Bellevue, but it is less aggravating as far as traffic goes (I usually go that way). However, for your purposes, I think you want to see the whole thing so head down to Paradise via the west side first and come back the east side. If you end up just doing the east side due to weather or you end up staying at Crystal Mountain, take I-405 south to the Hwy 169 (the Maple Valley highway) and take the exit to the right, continue straight to the second light and then left all the way to Enumclaw where you turn right on Hwy 410.
No bearspray. You very well might see bears but they will be focused on the huckleberries and won’t even notice you. Give them distance and respect. These are not grizzlies but black bears (even if they are brown). And also leave the bear bells at home; they serve no purpose other than to irritate your neighbors and to make sure you won’t see any wildlife at all.
The mosquitos should be dead by then. Just bring sunscreen/sunhat and an extra jacket in case it’s misty.
Bring a lunch rather than eating out. There are only a couple of spots to buy lunch and you will spend too much time waiting and not enough time enjoying the area. There are restaurants at: Ashford/Elbe, Longmire, Paradise, Greenwater and Enumclaw. In season, there is a small overpriced grill at Sunrise for minimal food in quality and choice.