At 18 mos. all we did was storytime at the library. I don't believe they need a lot of structured "activities" at a young age. Toddler attention spans are not the same length as older preschoolers/kids who enjoy 30-minute plus classes. We found that the things they do at the paid classes (Little Gym, music, etc.) were nothing more than stuff we already did at home and never paid for any of those. It might be a nice way to meet other families if you don't have a network of friends with same age kids though.
Sergio isn't in any formal activities although he's very interested in Hugo's martial arts and watches the classes and counts along in Japanese. I think he'll be ready in the fall, but has to turn 3 before he can enroll. Hugo's excited about him being in his class. We started Hugo there at 3. We believe in only 1 activity at a time at this age but might let him add some formal music practice/lesson in the future when he's older. Too many structured activities is not good... for your stress level or theirs.