In terms of what can be done at Erratic Rock Wayside, there isn't much selection. There is an approximately quarter mile paved trail leading uphill from the small parking area. You can walk up the hill and look at the rock.
This rock, currently about 40 tons or so (visitors are thought to have carried away some 70 tons over the years) was carried here by a glacial ice flow during the Missoula Floods some 20,000 years ago. This type of rock is referred to as a "glacial erratic" as it doesn't match any of the rock types around it, and has moved quite some distance.
Thus, the name of the facility: Erratic Rock State Wayside.
There is a picnic table near the rock at the top of the trail. Aside of the rock, there is a sweeping view of the Yamhill River valley and the surrounding hills. If there were actually some restroom facilities this might actually make a reasonable rest stop and picnic area for those who are able to make the short but fairly steep climb up the hill.
The trail right of way is about 10 feet wide, and there are bushes and small trees on both sides. There seemed to be a number of birds in these bushes, but all of them were fairly shy. One of them was certainly a woodpecker, most likely a downy woodpecker. There was at least one spotted towhee, a few song sparrows, a northern flicker, and possibly a bewick's wren, though he hid himself very quickly. Some of those were seen in the field of Christmas trees growing in the field next to the trail near the top. The woodpecker was in the one fairly large tree along the trail, near where it makes a sharp turn up the hill.
There is very little shelter at the top of the hill, so on a hot day this probably is not a good place to spend much time. With a lot of wind in winter, it would probably be quite cold. However, on November 15, 2008, it proved to be a perfect day for this type of thing: clear in order to see the entire valley, and not too hot, and not windy at all.
It isn't exactly the most interesting thing to do along highway 18 these days (I can remember a time, that doesn't seem that long ago, that I made trips to Lincoln City when there wasn't an aviation museum in McMinnville nor a casino in Grande Ronde). However, it is most certainly the oldest (by some 20,000 years) attraction along this road to the coast.
Driving is about the only way to get to Erratic Rock State Wayside. People do ride their bicycles on some of the roads out here, but the roads are very narrow and people drive very fast. Therefore, you would take your life in your hands by using your bike on Highway 18 - but people do it.
So, the only recommended way, I can say, is to drive.
There are several different ways to approach this. However, I will give you my suggested way:
On highway 18, heading southwest from McMinnville, you will pass Masonville Road on your right, which is about 3 miles outside of McMinnville proper. About 1 mile past this road, you will see a small sign saying "Glacial Rock" and pointing to your right. DON'T TAKE THAT ROAD. The sign has white letters and is on a brown background.
Instead, continue southwest on highway 18 for a little over a mile further. There, you will come to the intersection with Oldsville Road and you will see another sign that says "Glacial Rock" and points to your right. TAKE THAT ROAD.
There are a series of turns in the road. You will see a house on your right that has a large car port like structure that appears to also serve as a trellace for a vine. The road makes a sharp left turn, and you can't see what is past it. SLOW DOWN because you need to park just past this blind turn.
As you come around the curve, you will see a paved spot on your right that is simply marked "PARKING". This is the only parking facility for the park. Park as far forward as you possibly can, as this will help you leave in the event that someone comes flying around the blind curve in the road you just went around.
You can park in this parking area coming from the north, as indicated by the signs further north on highway 18. However, there are several reasons not to do this. First and foremost, to get to the parking area and to leave the parking area you must cross oncoming traffic and park going the wrong way. Secondly, when leaving the parking area, you must check for both oncoming traffic around the blind curve in the lane you will need to cross and you will have to check behind you for traffic in the southbound lanes.
By coming at the park from the south, you only have to worry about the traffic coming around the blind curve, which is enough to worry about.
If you are coming from the southwest on highway 18, start looking for Oldsville Road just after Bellevue. This road is about 1 mile past this community, and turn left. THERE IS NO SIGN (at least not as of this writting) that points the way to the glacial rock for those coming from this direction. Then, follow the above instructions on what to do after you get onto Oldsville Road.
For the official state parks department web site and phone number, see the Things to Do in Erratic Rock tip (not that there is that much to do here, but all both of them can be discussed in one tip).