I don't think October is a necessarily poor time to visit, aside from it getting darker earlier than in the summer. The weather in October is really not much different than in June or August.
What you want to do depends on what you want to get out of the trip. I live in a large city (more people in a one-block radius of my apartment than live on all of Shetland), so I'm attracted to the emptiness and solitude of the place. For me, it took several days to decompress and be able to enjoy the emptiness.
I liked the lack of things to do, aside from walking or bird-watching or the like. Just having a few days to do nothing, aside from walk, read, and play board games in an extraordinarily beautiful place, sounds like heaven to me. If you are from someplace more like Shetland, then maybe that's not as appealing.
I was there for 6 days (down from an originally planned 7, due to flight cancellations) and it seemed much too short. This summer's trip will be 10 days; I wish I could stay longer. The aim this year is to be able to explore at a more leisurely pace and just enjoy staying in one spot (specifically, Eshaness) and reading, lounging, and walking. We went on an otter watching tour last year, which was fantastic; this year we're still mulling over other options. Maybe going to Noss for a day (we didn't see puffins at all last year) if the weather is good, or maybe taking a boat tour. I'd like to walk Hermaness on Unst, particularly because it should be free of sheep poo, unlike the rest of Shetland. Jarlshof or tracing some of the Viking archaeological finds on Unst are also a possibility. I'm looking forward to visiting the
Shetland Reel distillery and
Valhalla brewery (both on Unst). If you're on Unst, you might as well stop by the
best bus stop anywhere.
Northmavine is probably the most spectacular natural landscape I have ever seen. I'm pretty much returning to Shetland so I can go back there, sit on the side of the cliff, and stare into the sea for hours with a book. The photo doesn't do it justice.
Also, it does take some time to get from island to island, so if you are staying on Mainland Shetland, and want to go Unst (or Noss or Papa Stour), you really need to plan a full day to accommodate trips to different islands.
The Visit Shetland web site mentioned above really does have the best overview of what you can do.