Iran can do plenty, it's already doing a shiton in Yemen, Syria and Iraq, Iranian influence is the greatest threat to the US in the middle east. The Quds force alone made Iraq hell for the occupation forces in Iraq. If the US commits to an all out war with an invasion it will be worse than Vietnam a thousand folds, if it commits to strikes Iran can retaliate by attacking oil tankers, infrastructures, escalating in Yemen, in Iraq, Bahrain which is a majority Shia country with a Sunni dictatorship where the American fifth fleet is located, or destabilize the Saudi eastern Shia regions. Iran isn't just Iran, it's also Yemen, Iraq, Syria, Lebanon.
When people ask what can Iran do, I keep reminding them of two things the Karbala raid conducted by the Quds force, and the 2006 Lebanon war.