Walks and play outside are excellent for this age. They are fascinated by leaves and rocks and bugs and the fresh air seems to stimulate them. For indoors, I'd say games like peek-a-boo, "this little piggy", and singing to and with them.
Being with their parents while they go about their daily activities in the real world. Being allowed to hold, move and touch *most* of what is in their world. Eye contact and smiling when you catch their eyes.
At this age, babies need several stable, significant people in their lives, food, shelter and basic hygie ne... and almost nothing else.
I use those wooden shape puzzles they make them for 18 months +, blocks, flash cards with colors or shapes on them, singing songs together like itsy bitsy spider, wheels on the bus, ect. , let them "help" fold laundry, pick up toys, put away silver wear, set up play dates with other children, playing outside
Great ideas above and also let her/him play with plastic containers that he/she can fill with small objects (nothing that he/she can choke on). Plus pot and pans so he can mimic cooking and stirring. Use the Swiffer when you sweep, etc. Basically anything that you and your SO do.