Oh OP, I feel you! I had similar aged DC last year.
There have been lots of good suggestions so far, like the 'Useless Shopping trip' (we did so many of those!), messy play etc.
For me, baby would be down for a nap from approx. 12-2pm, which left me with 3 hours until dinnertime after her waking up. The two hours of nap time were spent with DS aged 3.5, and it was a lot of him playing in his room with his toys/colouring etc. while I got laundry and other tasks done. He liked to listen to a podcast like Thomas the Tank Engine while he played, which was a nice change from the screen. He would sometimes help prep dinner, though I use the slow cooker and rice cooker almost everyday, and generally do my meal prep in the evenings.
Once DD was awake, it was snack time and then we HAD to go outside. If we didn't, we'd go stir crazy. Even in the rain etc. My DD also hated the rain cover, so many afternoons were awful. I purchased a puddle suit, some boots (once she was walking) and just let her get wet and dirty. Same for DS. I had several coats and suits that I rotated as they were always soaking wet. I also wore DD in the baby carrier (front facing) if she was really fussy, and we all just got soaked.
Once home, it was all about warming up with a warm drink, and then we'd do some activities together, like dancing to YouTube videos, or I'd bring some toys up from the basement. There is a great second-hand toy shop near me, and I'd get a huge box of toys for a tenner, and donate them again once we were sick of them. In our basement I have 7-8 boxes of toys ready to bring out on days with terrible weather.
This was all before we had a garden. Now that I have a garden, we are out there a lot in the afternoons. My DS likes to draw with chalk on the pavers, or play with his toys in a bucket of water, and DD just kind of toddles about.
But one thing's for sure: I never got anything 'done' in the afternoons. So I had to make sure all the chores/dinner etc. was sorted before DD woke up from nap. I learnt that the hard way!