Give two examples of things that you think are certain as can be. What makes them so certain?
1+1=2. This is a notation-independent mathematical fact. While humans have come to observe this truth using the senses (which is therefore doubtable), I trust that it exists independent of a perceiver (although, I do suppose I have no way of proving this outside the realm of the senses).
I perceive. I may not be able to be certain that the objects that I perceive actually hold the attributes that I perceive, but I (think I) can be sure that I perceive something.