If we're going to be so lackluster-ly broad and generic, I suppose the only real response would be: "that you might be wrong."

What can I be absolutely certain about? — Kranky

Given that you are not an "absolute" being (or clinically neurotic/paranoid), imo you are "certain about" whatever you lack rational grounds to doubt.

It would be possible for the thinking to exist without a thinker. — Kranky

Thinking is a complex topic, but we can for sure say that it exists. Thinking, however, is a coherent mental phenomenon. To develop a thought one needs to constantly retrieve memories of thoughts accumulated in the past, and work on thoughts to produce a new thought. Thoughts therefore must be memorized somewhere. Idealism cannot provide an answer for where thoughts are stored and how they are retrieved when it is appropriate. Therefore, idealism is false.

Even if my own existence cannot be proven, can awareness/experience be proven for certain? — Kranky

Experience for sure exists and everybody agrees with its existence.

Right now, at minimum, there is a belief that I am existing, a belief that I am typing this post.
I might not actually be typing this post, nor am I certain to exist.
But it is certain that there is a belief that I exist. — Kranky

The reality is coherent. Therefore, Idealism is false since it cannot account for the coherence. Therefore, you are a person who is typing your thoughts. By person, I mean that you have a body and at least two minds, so-called the conscious and subconscious mind.