Going to the colosseum is a great way to spend the afternoon.  You can walk around the outside of the building to get a feel of the scale and then head inside.  Once inside you can continue all the way to the end of the corridor if you need to take an elevator to the top.  There are plenty or stairs to get you to the different levels of the colosseum. 

You can walk around the inside of the colosseum but are not allowed into the center.  They have a platform that you are allowed to stand on that goes part way into the center.  This will give you great views of what the gladiators would have seen.