Sen. Al Franken really asks the central question here. Obviously, it could be any number of activities. But if I were to pick one especially egregious activity, it would be money laundering in coordination with various mob connections, both American and Russian. Here's one very detailed example by Todd Gitlin. But there are many more examples.
Now I know money laundering has nothing to do with Russians meddling in our election process, but it does fall under the "high crimes and misdemeanors" associated with articles of impeachment. And the optics of a President of the United States doing business with the Mob? Well, that's just killer.