'play with him/talk to him/entertain him/teach him'
Please don't put so much pressure on yourself. It is a myth that babies need entertaining, I promise you. At his age he just needs very few age appropriate simple toys to play with as he wishes (stacking cups, an o ball, and other really simple toys are best), some floor time, outdoor time where possible, cuddles, to be fed and cared for, opportunity to sleep, and given your full attention when you're doing something for him (e.g. Feed or nappy change). Talk through these activities saying what's happening.
Toys don't need to be rotated, just added to occasionally as they develop and show interests. They don't need baby groups, though by all means go if you enjoy them, but balance with lots of quiet time for the baby to wind down. No need to prattle on at them, just talk about what he's doing or looking at or you're doing for him, at regular intervals. Babies learn through play and watching, no need to 'teach'.
Completely agree re having a predictable routine, it will chop up the day for you and help him to feel secure as he'll learn what's coming next even before he understands all the language. E.g. Wake feed independent play, nap, feed, outdoor time (group? Walk?), nap, wake feed, independent floor play, bath, bed.
Literally everything is very stimulating to a young baby - a trip to the supermarket is a huge adventure.