Lithuanian here.

1) How much time on shore do you have?
2) What are you interested in?
3) How independent are you? Can you walk well?
4) Does the cruise company offer any tours, ideas, etc?

Time: About 8 hours in each port
Entire party (6) are mobile but not for long periods of time
Interests; food, natural beauty, national parks UNESCO
Trying to avoid using canned Cruise excursions

Although you are a small party, 8 hours is not much to work with considering your interest is „Interests; food, natural beauty, national parks UNESCO”. The things you desire require a ferry crossing to the Curonian spit. If you open GoogleMaps and input “Klaipeda cruiseship terminal” and “Smiltyne”, you will have a picture what is going on.

Your best option would be hiring private transfer to the Curonian Spit and be driven around the UNESCO heritage of Nida and the surroundings. You can do it on your own with public transport, but with 8 hours and 6 people who aren’t mobile for a prolonged period of time- it is a bit risky.

I suggest you look at https://www.visitneringa.com/en and let me know if this is something you would like to see?

May I inquire what option is offered by the cruise company? Are you aware of their pricing? It may be worthwhile to consider their offer.

In Klaipeda jump on the frequent little passenger ferry at the port entrance, across to the Curonian Spit. Service buses meet the ferry on the other side. No idea of guides over there, very easy to do on your own. I, and the ferry, get very busy in summer, especially at weekends.
No booking, turn up and go.
It is narrow, but around 60 miles long, a UNESCO world heritage site- https://whc.unesco.org/en/list/994

The ship will probably offer a tour, which will go over on the car ferry from a different part of the City.

isn31c,

your offer is not bad, however, you are talking about taking a public transport bus. If it is first come-first served and if it is full, boarding may be denied, they are not free buses. In summer it may well happen. I'd suggest booking tickets online but then it is not independent, no guiding, and valuable time will be lost walking around a small portion of what can be discovered. I would second your suggestion if the OP had a full day, for 8 hours that is just risky, especially if the return bus schedule does not work for their group.

We did a tour of Russia and all three Baltic Countries in 2011. All three of the Baltic countries had the friendliest people. They loved Americans.
Riga is a beautiful and historic city, suggest taking a tour of that city.
We didn't visit Klaipeda, but loved Vilnius, Lithuania, not sure how far it is from the port.

Vilnius is about 4-5 hours by coach/train from Klaipeda, by car much closer.

Foodwise, Latvia and Lithuania are quite similar.

Since you will be by the sea in Lithuania, any kind of fish soup is a good idea. It is often made from the catch of the day. Also, in summer our famous pink cold beetroot soup is very popular. Lithuanian beer is good and Klaipeda has a few breweries that also do tours. Klaipeda area is famous for the birthplace of „Dziugas“ cheese- a type of local parmigiana. Try “Dziugas Cheese House” locations in Klaipeda, they even do cheese ice-cream. Icky, I know :-) In general, our diet is very meat and potato based, it may not be to everyone’s liking. “Etno Dvaras” is a chain that offers traditional Lithuanian food.

I don’t know if you are allowed to bring food from outside to the cruise ship? A bigger supermarket in Klaipeda (not LIDL, try RIMI or Maxima, IKI) will sell good Lithuanian bread, Lithuanian cheese (try our curd cheese and its varieties), Lithuanian sausage (try “skilandis” crazy expensive but so good), dessert Baumkuchen (“sakotis”, a tree cake), “Ruta” or “Pergale” confectionery products, “Voruta” fruit wines (national heritage certificate), dried porcini mushrooms (local produce, we are crazy about mushrooms), smoked fish (seaside local fare)...