wehre to go and what to do for a few days on the High Coast of Sweden

Our current thoughts are to take the train from the Arlanda train
station to Gävle in the evening of the 14th and stay the night there.
Then the questions begin! Should we stay more than that night in
Gävle, or take a train farther north in the early afternoon of the
15th? Would it be better to go all the way up to Örnsköldsvik, or stay
in Härnösand?

What time to you arrive at Arlanda? Going north of Gävle from Arlanda makes things a bit more complicated. It's easy to get to Gävle from Arlanda by regional train and there is no need to buy the tickets in advance. If you're going somewhere north of Gävle you either need to change to another regional train in Gävle, or take a high speed train which should be prebooked.

However, I'm not sure I'd suggest heading to Gävle the first night. Uppsala is a more interesting place to spend the first night, even if you are going there later. Although you can certainly spend some time in Gävle without being bored. More information on your trip would make it easier to give advice.

Are there guided boat rides around the islands? Or guided kayak
rentals? Where would you recommend we go to? We aren't super athletes
or anything, but would like to play around outside as well as seeing
some culture and history.

A good start is https://www.hogakusten.com/en where you can find a lot of information about the area. You can rent kayaks but I'm not sure there are any guided tours. Although I assume that any place that rents kayaks will be happy to provide you with maps and information about the area.

There are boat tours, as well as regular ferries to some islands. A visit to Ulvön is not a bad idea and I recommend spending a night there. Apart from being a charming island, the ferry trip there also gives you a great view of the archipelago. And Älvturen, https://älvturen.se/ , have guided boat trips on the Ångerman river. And if you enjoy nature, don't miss Skuleskogen national park, https://www.sverigesnationalparker.se/en/choose-park---list/skuleskogen-national-park/

Also, if it is convenient to get to sights in the area, we would be
happy to just travel by train/bus, but could instead rent a car while
in the area, if it would give us a lot more flexibility or options
(opinions?).

A mixture would probably be a good idea. Trains and buses are a good way to get between the towns. As well as to smaller towns, and to the ferry to Ulvön and other places. But a car can be useful. If you're e.g. going to Skuleskogen, renting a car for a day will give you more flexibility.

Thank you Badger for the suggestions and the information that we do not need to reserve seats on the regional trains. Our flight is supposed to arrive at 5 pm and since I will later have more than 3 days to tour around Uppsala while my spouse attends a conference, I thought being able to see a bit of a different town might be nice, even though it won't be for long before heading farther north. I don't want to be optimistic about how much energy we will have on the first evening after our transatlantic journey, and Gavle seems a little less expensive than Uppsala for our first landing spot. We would continue our train journey the next day to destinations north.

Yes, we would like to go to Skuleskogen national park and also taking the funicular up Skuleberget hill to take a walk there looks like it might be a good option, so we will definitely rent a car. I have looked at a good bit of information on the Hoga Kusten website and the boat trips on the Ångerman river as well. We tend to like to have a home-base and venture out from there instead of having to constantly pack up and move, so I thought that maybe Örnsköldsvik, staying at the First Hotel Statt might be a good location for a few nights. Does that seem good or would there be a better town? Are there places north of Örnsköldsvik that we might want to venture to for a 1/2-day or day trip?

We try but haven't been very successful at applying Rick's recommendations of light packing and the vagaries of the weather and need to bring a few host gifts for our later visits with friends mean we will have more than a backpack, unfortunately!

Another question! Should we stay in the high coast region for 3 days or, since we are going to rent a car for some of the time, would there be a good reason to drive inland and maybe go to Östersund and stay there for a night? Or some other place that would give us a different experience? After the conference in Uppsala we will be staying on an island for a few days to celebrate Midsummer with a friend, then in Linkoping for a couple of days and then in Stockholm for another couple of days before coming back home. Thanks for any advice people can offer on visiting some places that don't make it into the "highlights" lists.

This may not be what you're looking for at all, because it's an urban rather than rural experience, but I loved a full day in Sunsdvall (south of Örnsköldsvik), mostly just drinking in the Art Nouveau architecture. I hadn't heard of the place two years ago, so I was lucky that Badger mentioned it in a post before my trip. It was voted the most beautiful town in Sweden not too long ago. It has a population of 60,000.

Here's Badger's post: https://community.ricksteves.com/travel-forum/sweden/and-the-most-beutiful-town-in-sweden-is

Wonderful information & link. Thank you! We will at least spend a few hours in Sundsvall! I noticed that Härnösand is also on the list, so maybe we will stay there instead of heading farther north to Örnsköldsvik.

Thanks for the additional information! But maybe your spouse wants to see a bit of Uppsala as well? But Gävle works as well. You should not go further north in my opinion as it will be a very late arrival. And you can probably spend half a day in Gävle without feeling bored. Unfortunately the biggest attraction in Gävle, the national railway museum, is closed for renovation.

It is a large area, so by having one base you are limited in term of how much ground you can cover. But for Höga Kusten I'd say Örnsköldsvik is a very base if that is your cup of tea. Close to both Skuleskogen and Ulvön. Sundsvall is a well worth a visit as well. Härnösand is also worth a visit and would work as a base, but is a bit further from Skuleskogen and Ulvön. Regarding places to visit north of Örnsköldsvik, Umeå is worth a day trip (1 hour by train). You can certainly spend 3 days (I assume that means 4 nights) in the region, but going to Östersund for one night would not be my first suggestion. It's a 5 hour drive one way. If you're looking for another town to stay in, make it 3 nights in Örnsköldsvik and one night in Sundsvall on the way south.