In addition to the bookstores, almost all of the libraries in the area have free storytime sessions for infants and toddlers. Those are great because they incorporate music as well as stories. In the summer, the libraries begin their annual summer reading series. All branches host special programs for children of all ages. You should be able to find literature about those special programs by May or June.
Chuck E Cheese on King St. should have a toddler zone. (I haven't been to that one in about three years, but most of the restaurants have a toddler zone now. There is no cost for admission. Only for the games and food.
Be sure to check out other family-oriented special events in the area, like the Kennedy Center Open House (September) and the Smithsonian Folk Life Festival (I think it's June). While definitely over your 18-month-old's level, they are fun and educational for everyone. The Folk Life Festival often has a sensory station based upon the theme. One year, children could touch corn, soybeans, and barley.
Also, check out local farms, the zoo (DC), and nature centers, which all have either free or very nominally priced events for toddlers. Do a Google search on petting zoos in Virginia, MD, and DC, and you should come up with something. (Be sure to sanitize baby's hands after petting livestock.) The Clearwater Nature Center in Clinton, MD, and the Watkins Nature Center in Largo, MD, often have great programs for children 2 & up for around $2.
Finally, even a trip to Petco or Petsmart can be very interesting to a baby. Don't forget that everything is new to your baby. The grocery store, the post office, the fire department (usually a group preferred), are all great places to visit once in a while.