This is one of my most favorite hikes in the Northwest; the views at the top are amazing. There really is a mail box at the top. You can sign the box or take a note up to drop in. This is a very challenging hike, you need to be in very good physical condition to make the attempt, on the order of hiking to Camp Muir, another one of my most favorite hikes; it is a reward all by itself just to make it to the top. Early spring may have lots of snow the last half mile up; I usually carry my Kahtoola micro-spikes and trekking poles. The new mailbox peak trail is not as demanding for those not wanting quite the challenge but still the great views on top.
Take exit 34 from I-90. Head north on 468th Ave SE for approximately half a mile until the intersection with the SE Middle Fork Road. Turn right onto the Middle Fork Road. Follow SE Middle Fork Road 2.2 miles to the stop sign at the junction with SE Dorothy Lake Road. Head left onto SE Middle Fork Road and drive .3 miles. Take the turnoff to the trailhead on the right and continue a short distance to the parking lot.
Bring a note to drop into the mailbox