In Iceland late Sept, early Oct.... What to do?

I will be traveling to Iceland late Sept and I don't know how to plan our 7 days. There seems to be so much to do.

Well actually that's the beginning of the low season, and I think you'll find that the range of tours has substantially reduced from the high season by then. Some snowfall in the interior can be expected in September, and usually some substantial parts of the interior have become inaccessible by the end of the month.

I suggest googling some tour companies and seeing what actually is available at that time of year, and what tickles your fancy.

Not sure if I should book tours before I arrive.

Not essential for the standard fare.

tips on being cheap

Hire a car and do it yourself rather than going on tours, unless you are wanting to do stuff requiring equipment, boats, animals, etc. Bring sleeping bags and stay in SBACC (sleeping bag accommodation). Self-cater.

what's the voltage in Iceland?

220V, as for continental Europe.

What are some attractions we should visit?

Everyone is different. Buy a colourful guidebook, or borrow some from a library, or google people's phototravelogues, and see what you fancy.