My family and I will start out in Seattle heading for Astoria and then down 101 to San Francisco.

We are travelling in mid August, during the week. Do you think I should make reservations in advance for the coast hotels/motels? Would Coos Bay be a good first nite stopover? We plan to take our time, walking on the beach but also we are use to travelling.

Orginally thought of going to the Newport aquarium but now am opting to go to the Monterey aquarium instead. Is this a good idea?

If it's during the week and you don't mind staying out of sight of the actual beach (Coos Bay for example) you shouldn't need to over-plan your stopovers.

Coos Bay is around 7-8 hours driving time from Seattle taking 101 all the way from the I-5 junction in Olympia. If you want to stop for strolls or photo ops (many opportunities on this route) plan either on an early departure or a late arrival. If you get behind, Florence might make a good alternate overnight spot to Coos Bay.

If the weather's fine, don't miss the chance to hit the beach just south of the Heceta Head lighthouse (a few miles N. of Florence.) BRING YOUR CAMERA for what's probably the best view on the coast. Also keep a sharp eye on the road - I had to hit the brakes a couple of weeks ago just north of Heceta because a black bear was crossing 101 - first time I've seen a bear on this road.

Thanks for your comment and advice.

The Monterey Bay Aquarium is always a good idea. I grew up in the area and have been to the Aquarium about 20 times. I enjoy it every time.

FYI
Astoria to Coos Bay about 230 miles.
However,because of traffic congestion usually June thru August you must allow about 5 hrs. Of course it usually amounts to more because you want to stop for the scenery. Maybe even Ice Cream in the Cheese factory at Tillamook???
From our experience Coos Bay to Eureka= 4 1/2 hrs. and Eureka to
SF 5 hr.
Hope this helps.
P.S.
I also vote for the Monterey Aquarium

What do you think of prebooking hotel/motel rooms along the Oregon coast during the week in August?

I have heard Lincoln Beach in Oregon is nice. Does anyone have an opinion on Lincoln Beach? Whether to visit for a few hours or a stopover for the night?

I would suggest staying in Lincoln City, then heading down the coast to Gold Beach or go a little further to Arcata, CA.