If you are planning on doing this as a round-trip, you may not have too much time to linger in the lower river. Trip down will be fast, since the current is now at 2+ kts. Tides will start to help or hinder you once you get down past St Helens.
You can tie up free at the docks at the island across from the St Helens waterfront. You could anchor in the lagoon at Martin Island, but I would not advise swimming.
New docks to tie up to at Kalama, but there is a fee. Also, there is a nice McMenamins restaurant there.
Free protected float behind Walker Island which is owned by a YC but specifically allows others to use it when they are not.
Cathlamet has a totally protected marina, and showers/fuel. Without local knowledge, best to go around Puget Island and approach from downriver.
All new public dock (!) up inside the Westport Slough.
Great anchorage in the huge harbor east side of Tongue Point, by inside and Mott Islands. Sand/mud bottom, 18 feet uniform depth.
West Basin at Astoria is nice, has fuel, and showers are probably still free. Call them when approaching, for slip assignment.
Best plan for about three days returning home, or two fairly long days of listening to the engine. Sometimes you will get a fresh breeze to sail up the river, in the afternoons. Main river is fast and cold now, and swimming might be ill advised. Hypothermia is very real.
Lots of scenery to enjoy, and we will probably be doing a similar trip later this summer.
Where do you moor? We sail out of RCYC.