We have a trip to Greece in less than a month - early October. Any speculation on impact if Greece defaults?

Not much I'd have thought.
Merchants still need customers, and hotels and restaurants their guests, and they will be even more eager to attract high-spending overseas tourists. There may be some disruptions to transport and other services, as workers go on strike and join anti-government, anti-EU demonstrations. But life still has to go on.

You might get a better exchange rate on the euro. ;-) Apart from that, tourists won't be affected at all.

If it impacts the ability of the government to continue to pay people or forces them to, for example, raise the retirement age, you might see some rioting and strikes. Probably not too bad, but Athens might be a bit of a mess in spots.

I am in Greece right now, and there is no discernible effect on the tourism industry at all. There may be a few more strikes as workers come back from their summer holidays, but nothing to be too concerned about.