Firstly, although it seems tricky, get out of the flat. Going out with baby in a sling or pushchair will help them, and it'll also help you
You don't need to be doing lots of activities with him, the world (and your face!) is plenty. Obviously do it if your want to, but don't feel like you need to.
Go and do things that YOU want. Go for a walk, go shopping, have a wander round a museum, meet s friend for lunch, it to to one of the billion baby classes around that are frankly more to keep you entertained than the baby at this age. Through things like baby classes and playgroups your can meet some friends in a similar boat, and meet up for a pub lunch or something, or hang out round eachothers houses.
But do what you fancy, but what you think baby wants. You've got years of going to soft play and hanging round parks freezing your tits off, so make the most of the time when they drift care what they do as long as it's with you.
Maternity leave can feel very lonely and restrictive, or it can be a time of reconnecting with friends, going to new places without the restrictions of a job. Ok, with baby, but you'll soon get used to them ragging along.
It's early days as it's the first week alone. Maybe think of a few things you fancy doing next week and then see us you can do then in a way your baby can tag along with. A Christmas market maybe?