Baby carrier, good weather, free time.
When your baby is about 6 months old, take him on a tactile tour of your neighborhood. Put him in a Baby Bjorn or some other type of carrier (I found this easier than a stroller) and take a walk, pausing at points that might be interesting to touch for your child. Tree bark, lamp posts, leaves, branches, metal sign posts, and much more will be a sensory adventure for your little one. Just be careful of any plant life that could be toxic and be sure to wash your little one's hands once arriving home. This adds a brand new perspective and delight to your walks about the neighborhood, helps to develop sensory perception, and builds vocabulary.