Add up items aloud as you perform daily activities, while including your preschooler in the conversation. For example, when setting a table for four place two plates on the table, count them aloud, and say "Two plates. How many more do we need to make four?" Then reinforce that you need two more plates, because two and two make four.
Count and add toys as your child plays, cars in parking lots on a walk, birds, people, clothing while folding, cups while pouring drinks and whatever other objects present themselves in the course of daily activities. Keep an eye out for anything that occurs in small clusters, which you can count separately before adding them together.
Reinforce the lessons with more focused crafts and activities. During snack time you can group about ten berries or small pieces of fruit into a few piles. Ask "How many berries are over here?" and count them together, then point to another pile and ask "And how many are here?" and count those together. Combine the piles and tell your child something like "Three and five make eight. See?" and count up the eight pieces in the new combined pile. Crafts like making bead necklaces, pasting cutouts to paper or building structures out of Lego, Duplo or wooden blocks can also serve as opportunities to practice addition.