'a lot to say' and 'a lot of things to say' seem correct to me.
I'm not a native speaker of english; However, I'd use 'a lot to say' when I'm intending to mean "I haven't got a lot of stuff to speak" I'd rather omit 'of stuff', and use only 'a lot'. Stuff is immeasurable, and I can't say 'many stuffs'. For example, if one's proficient in sports, and can speak at length about soccer, he