A wonderful trail! At the south end eter from the Arcata side, you are greeted by the Mad river. Enjoy the Arcata bottom area, before entering the Hammond trail.
Now in Mckinleyville, a short hill after the mad river bridge, then it is flat for a long ways. Not too long up the trail you will reach Hiller park - picnic tables, kids playground, single track trails, batting cage, baseball fields, bathrooms, and probably more.
Just north of the the park it's a pleasant area with lots of tree coverage. Shortly you'll begin to see ocean views. The farther north you go the more intense, and closer the ocean access becomes. Most areas of the trail will have tree coverage.
A few hills exist along the entire trail, but they are very manageable. You also have numerous access points with parking areas, if you prefer to drive then walk segments of the trail.
The far north end is Clam beach, you have full access to the beach from this area. Actually from Hiller park you can access the mouth of the Mad river, but you need to be more familiar with the area, to know what single track trails access the area off of the Hammond trail.
One segment of the trail north of Clam beach, the trail is gravel. Though a short segment everything else is paved. Be prepared for a bit of a hill in that gravel area, but it offers a wonderful overview vista of the ocean, and you can even find a bench halfway up the hill. North of the gravel it's flat and a bit windy with dunes to your west. It's en exciting stretch as you know you're reaching Clam beach.
My experience is riding a bicycle from downtown Arcata, and sometimes all the way up to Clam beach, then back. Not matter how often, or far I ride the bicycle, it's always a wonderful, and relaxing ride. At Hiller road just south of Hiller park head east to the main part of town. If you would like drinks, food, ice cream, etc. You'll find all you need at the foot of Hiller, just a short ride!
Enjoy the coastline!