Clarisse and Mildred are not similar in many ways, and Bradbury most likely did that on purpose. Mildred was a symbol of their society's mindlessness, and Clarisse was a symbol for what their society is missing, the society of thinking, living, and being happy. So, to find similarities you have to go down to the basics, like, they are both female, they both play a significant role in Montag's life, and they both act as catalysts for Montag to change (Clarisse to initiate change, Mildred to leap over the cliff into full rebellion as she calls the alarm in on him and abandons him).
In activities and beliefs, they truly don't have very many similarities. It would be easier to discuss how they are different in those areas, because they are, completely. They both believe in Montag, though for different reasons. Clarisse believes he is a kind man, who cares to listen to her, which means he is different, and a good human being. Mildred believes in Montag's steadiness, and his ability to provide a living to support her lifestyle. So, the belief in Montag is there, just in different ways. They both believe in doing things that please them-for Clarisse this is thinking about the why of things, talking with her family, and asking questions. For Mildred, it is watching t.v. and socializing with her friends. In that way, their activities are similar in the fact that they do things they enjoy. They both enjoy driving, although Mildred likes driving when she is upset. She tells Montag that "I always like to drive fast when I feel" upset. Clarisse on the other hand, likes watching people drive "on the boulevard" and wonders how they "know what grass is, or flowers, because they never see them" because they are going too fast.
So, there are some ideas for you; it's a tough question, that's for sure! Good luck!