You can have your baby around more people without joining a baby group. You could maybe add some more things to your routine: taking the baby to the grocery once or twice a week - show them the fruits and vegetables, tell them the names of things. There will probably be a few elderly women shopping at the grocery store who would be more than delighted to say hello and wave at your baby.
You could take your baby to the park once or twice a week and look at the ducks and other birds, or watch other children playing on the playgrounds. Children love babies and will likely bring things to show your baby - blades of grass, twigs, pebbles, their toys, etc.
You could take your baby for a ride on the train or bus and your baby can watch the buildings and cars go by outside the window.
If you get a weekly garbage pickup, many babies love to watch garbage trucks! Or, any regular thing that happens around your flat. If the mail usually gets dropped off at, say, 10 am, you can make sure to be down in the lobby at 10 am so your baby can see the mailboxes being opened and the mail put in. The mail delivery person may find you a bit strange at first but after a few days will probably enjoy waving at your baby, although will not have time to stop and chat.
You can listen to music that YOU like with your baby. It doesn't have to be baby music.
You might increase your reading time to two 15 minute periods per day.
Your baby will also enjoy watching you make an egg. You can point to the egg and say, "Egg!" When you make a sandwich for your lunch: sandwich, bread, ham, slicing, etc. It is good to narrate what you are doing as you work, babies love that. "I am going to fold the laundry. This shirt is red. It is my favorite shirt." It's good to talk in baby-language (high voice, simple sentences) as babies learn language better this way but any talking you do is great.
But overall, if you and your baby are happy together, that is the important thing. Don't pack so many activities in to your day that you become stressed out, as the baby will feel that and will respond to it with his/her own stress. Happiness is good for baby brains.