You 100% need mum friends/regular groups! I get really depressed and feel like groundhog day when I don't have this.
I have 3 groups I go to regularly - a coffee meet up every Wednesday, then two which run on Fridays so I alternate weeks.
Tend to also try and meet up one on one with a friend or family member another day of the week, so that's 3 days socialising. For a meet up I would invite someone to my house, or we would go to theirs, or if it's nice we might meet at a park, if rainy then the pool, soft play, etc. If you don't know anybody to do this with, the groups (or even paid classes) are important. Try to go to the same things regularly so that you get to know the people who come all the time and make an effort to talk to people and remember things about their kids and ask them about them. That can then lead to being added to whatsapp groups or just asking somebody one on one if they want to have a coffee or visit you at home.
Then on another day I'd run errands/do something out of the house like take toddler swimming on my own or visit local museum, let them wander around IKEA etc.
The meet up/activity would take up half the day (generally the morning).
In the afternoons/on my home day, I tend to try and do some basic cleaning jobs, or sit on my computer with a coffee, while toddler plays with toys. We might go for a walk to local woods or park. (We have waterproofs but I don't go out on every rainy day). I might help him build a train track to play with, or I have a tub of old rice/quinoa (dried) that we use as a kind of sand game to practice scooping and pouring. Water play on the balcony if it's hot. Make obstacle courses in living room/bedroom. Simple board games (2.5y+), colouring, playdoh (3y+), glue/cut/stick (3y+), reading stories, TV, marble run (18m+), jigsaws.
I have toys they can play with alone, whcih are always accessible, and activities that I would help with or supervise more closely, which are kept higher up or in closed cabinets. I would sometimes set up some of the "play alone" toys, or just leave the drawer (etc) open.