if you are planning to go to downtown chicago

you don't need to rent a car.  Parking in chicago downtown, around the millenium park area isn't cheap; $20, 30 for a day.  Once you arrive at the o'hare airport, just take the blue line to jackson(i think it is jackson, i might be wrong, just ask around when you are get there), and change to red line, it will take you directly to chinatown.  And you will be able to go to millenium park/navy pier(must sees if you come to chicago) by taking the subway as well; the michigan lake and many of the museums are within walking distance of the millenium park. 

weather wise, in april, the temperature in the low 50s, the best time to go to chicago is in the summer, there are concerts held at the millenium park on a regular basis during night time.  You can smell the fragrance of the grass and feel the breeze of the michigan lake as you walk around the lake, which is really nice.

Hope what i wrote helps and have a wonderful trip! :-)

shuo