Multnomah Falls is on a senic loop road and is the easiest of the four to get to and has a hiking trail. The one I like is Oneonta Falls. You hike in through the creek through a little gorge. It is very cool and your feet might get wet. Bridal Veil Falls is ok.
At the east end of the scenic loop you can head east on the freeway and head to Bonnieville Dam and check out the fish ladder and rearing ponds. They have a sturgeon pond with underground viewing area that is pretty cool. If the salmon are running the fish ladder viewing is a pretty cool sight.
If the salmon are running a guided fishing trip near Bonnieville Dam can be very fun.
In Portland there is the zoo, the gardens, the Grotto, the Oregon Museum of Science and Industry (OMSI ), Pitock Mansion, Portland Obsevatory , a bunch of other things on the water fronts, China Town and all kinds of breweries.
Portland is also know for its adult entertainment.
The drive to Newport can be fast using I=5 but way more senic heading from Portland to Tillamook and heading south on HWY 101. In Tillamook you can visit the Cheese Factory and the Air Museum if you want. Heading south take you through many coastal cities with many things to do and see. Before you get to Newport you would be going through Lincoln City where the Casino is located. Im not into the Casino dealio so I would be heading to the Embarcadero and tossing a line out for crab. We own Embarcadero through VI.
Newport is pretty cool. There is the Undersea Gardens and all kinds of tourist activities. Exploring HWY 101 beaches is what we usually do. You can rent a small boat and go crabbing or fishing but I think its more fun to sit on the dock or shore and use a crab trap on the end of a fishing pole. You can get all of the stuff you need at the Fred Meyers. Restaurants seem to come and go but near Lincoln City is Fantoms at the Spanish Head with an ocean view.
Im sure other fans of this area will be chiming in soon.
Bill