Calvin once made Susie a valentine and flowers (albeit a hate-mail valentine and dead flowers, but it showed he cared).
In that same story arc, Calvin was left thinking "She noticed! She likes me!".
In one Sunday strip, Calvin and Susie exchange insults, and Calvin says "It's shameless the way we flirt".
Calvin's good side began writing romantic love poems to Susie and offering to carry her books for her and sent her flowers. As it is a projection of Calvin's good side, and therefore, is half of him, this practically confirms Calvin's love for Susie.
In one strip when Hobbes was stolen by a big dog, when Calvin found out that Susie had Hobbes, he kissed her hand, but Hobbes stole her cookies.
Calvin sits with Susie during lunch.
When Calvin calls Susie a "booger-brain" and makes her cry, he genuinely feels bad and is upset when she initially ignores his attempt to apologize.
Once, when he did not sign up for baseball, he played with Susie on the see-saw.
Sometimes he stands next to her at the bus stop, and sits with her in the bus.