Obvious thing is the various "Congressional insider trading" scandals that no one ever seems to want to talk about too much (what happens when the whole system is corrupt). Of course there are future job promises, not just for Senator Atrios, but for his entire network of family and friends. And, well, let's not discount giant sacks of cash maybe just being placed on the desks (that one might - might - be illegal).
For decades the entire Congressional impure thoughts conversation was about campaign donations. But, really, that's silly. No one is going to dance to the tune of their donors simply because they need campaign cash, something which is absolutely easy for incumbents to raise in other ways.