I also have a 15 week DS. For me it is my second maternity leave - I have a 3yo DD. My 3 yo DD was by far the harder baby and I had PND, but despite that this maternity leave is harder in so many ways, so really don't underestimate the impact of covid.
With my older DD we did groups and activities every day - I was always busy. It gave me structure and made the day go quicker. I saw lots of other adults and we could have a chat and share our struggles. This time I have been bored and lonely an awful lot. I have found myself obsessed with nap schedules because it's all I have to give me structure. Also, I know lots of people say little babies don't get bored but both of mine have been happier out and about from about 2 months, which I can only put down to some level of boredom. Which again is much harder this time with less places to go.
DS is my second baby and I am still rubbish at telling what his cries mean so don't worry - it gets much easier from that perspective when they talk!!
Things that do help me this time are:
-My older daughter does nursery 3 days per week, but the days she is home we go to the playground and DS loves watching the older kids play. Do you have a friend with a toddler/pre-schopler you could accompany to park? I guess it might feel a bit wierd taking your baby to the playground just to watch other children play!
-Lots of little trips out rather than one big one - so half hour walk first thing, then pop round the supermarket later, then in the afternoon takeaway coffee in the park.
-A couple of music baby group things have started up round us recently so I'm starting two this week. Will work out very expensive if we miss lots due to self-isolating or local lockdowns or anything, but worth it to keep my sanity.
-Dancing around holding him to music I like. Plenty of time for nursery rhymes when he's older (although I do quite enjoy the Spotify Disney playlist!).
-I give him a long bath early afternoon most days which he loves. A local family friendly swimming pool near us has reopened recently with really well thought through social distancing so maybe look into anything like that. Both my babies have loved water.
-I try and hang out in different rooms. Gives us both a change of scene! Especially useful if the weather is bad so we can't get out much. During the summer we put a tent in the garden for my daughter, but DS liked being in there to!
The mind numbing boredness will improve when they get older, because you just get more and more back from them, but also don't feel guilty if going back to work feels amazing. I absolutely loved years 1-2 with my DD because she was such fun to be around and developing so fast, but also I got three days at work to not think about baby stuff.