Our little one hated his car seat until we bought this activity bar thing that we clip onto the sides of the car seat. We got ours at right start. It is like an activity bar that would go on a bouncy seat but it can clamp onto what ever you like. As soon as we started using that, he was sooooo much happier. He is just a really busy guy, so I think he gets very bored and unamused when in his car seat. If we leave it in the other car though (therefore forgetting to have it clamped onto his infant seat) he is a nightmare!! The toys that you hang from the handle just did not do it for him. We also started linking a couple toys to the seat so he could play with those. He also likes to look in the mirror that we have in there so we can see him, but that alone is not enough to keep him happy.
We just switched to the next stage car seat, and you can't clamp that bar onto to those carseats. So we had to find a toy that would work for that kind of seat.
I have heard some kids just don't like certain carseats, but unless you have disposable money and want to purchase another one, try some really engaging toys first.