Need help in planning a trip to Germany and Spain

It sounds as if beach time is pretty important to you, so I suggest checking actual day-by-day temperatures for San Sebastian in late Sept. and early Oct. of 2017, 2016, 2015, etc. That data is available on wunderground.com. Choose History > Monthly. Also Google for water temperatures in the area.

Even in mid-summer there's no guarantee of what I would consider beach weather in the Basque Country, which tends to be cool and damp. By fall I suspect weather suitable for swiming may be pretty uncommon, though that obviously depends on what constitutes comfortable temperatures for you.

Yep, exactly what acraven said.

I would reinforce that with some additional cautions about what kind of a "beach experience" you might be expecting near San Sebastian. Not sure where you're from, but IME many Americans have a mental image of "fun at the beach" which looks like Hawaii, Cancun or Thailand - broad, open (largely empty) swaths of sugar-fine, white sand, lapped by clear, warm water. You might get some of that near Barcelona but I think the beach experience in many parts of Europe, including near San Sebastian, are not really like that at all. Not necessarily saying you shouldn't go there, but if you haven't been to that part of Europe before, just be aware of what to expect (especially in October!).

I like traveling by train, but check the schedules to see how long it takes. You'll save time (and perhaps money) when you fly if the train ride is longer than five hours (2 hours for the flight, 2 hours arriving at the airport in advance, 1 hour getting from the airport into town).

There is quite a lot to do in Barcelona. I was there two days in April and wished I'd allowed a week.

Planning gets trickier the more people you add. My husband and I went with our two adult children once and we had multiple family meetings trying to decide how many days to spend in each of several cities. What worked for us was to ask each person to bring three suggestions to the table and try to arrange it so everyone got to see at least one of their choices. It's also possible to split up on some days and go in different directions.

The average surface water temperature in Donostia / San Sebastián is decreasing during October, falling by 5°F, from 67°F to 62°F, over the course of the month.

https://weatherspark.com/m/40923/10/Average-Weather-in-October-in-Donostia-San-Sebasti%C3%A1n-Spain

I was in Barcelona in January and we enjoyed their beaches, tho we did not swim. I would think in September the beaches in Barcelona would be lovely. I did not get to the areas on the coast north of Barcelona, but I heard those beaches and areas are lovely. You could consider adding days to Barcelona/Catalonia.

San Sebastián has a beautiful beach but the average high temperature in October is only 67 vs. 73 in Barcelona. If you want beach time, the north coast is not too inviting at that time of year. The upside is the beach in San Sebastián will be empty.

To explain a little more, San Sebastian is on the cold Atlantic Ocean. The Med is still relatively warm in early October, don't know if the Atlantic ever is. Consider Valencia or Malaga, both pretty towns with some sights as well as beaches.

Keep in mind that, as it's June, you're quite late for planning an Oktoberfest trip unless you're OK with staying outside of Munich proper and taking a train in, or paying 3x normal rates for a hotel. Also keep in mind that you won't be able to reserve a table at this date, so your group will have to queue and search for space for such a large group inside. I'd highly recommend visiting on a weekday morning/early afternoon for an optimal experience.

(Or you could visit Stuttgart's Volksfest, which is basically just like Oktoberfest but far more reasonable to manage, and you MIGHT even be able to get a table reservation at this date.)

Your best option for travel between Germany and Spain is probably going to be a budget airline. Do be careful about which airports they fly in/out of, there are some airports that are in the middle of nowhere, so that cheap flight may end up being way more expensive when you factor in travel to the airport.