Here are some thoughts:
1. 'just sitting' can be very
powerful. I don't think that you need to find something to talk about
all the time. Maybe just packing a book when you visit, or a magazine,
and 'just sit' with him would be sufficient. Maybe during that time,
your dad will say something and you can pick up on that and go from
there. When you leave, may I suggest that you simply tell your dad that
you love him, or you are glad for your visit. Even if you dad says
nothing or is having difficulty communicating, remember that he might
still be able to hear well, and -perhaps- process information a lot
better than he can convey it, so being proactive with such comments can
build bridges for future visits.
2. Ask questions. They might help you get a better bead on where your dad is in his thinking.
Remember the things that your dad liked to do, and if possible,
consider being proactive by taking the lead and gently suggesting to
your father that you would like to do such and such.
When was the last time your dad had a complete physical? Perhaps you
might find out somethings that might help you to better care for your
dad if you knew more of why he is having 'good' and 'bad' days.