There are so many things that I love to do on my baby days.
Usually I start off by powdering and diapering up, then I throw on one of my onesies or one of my rompers. When I'm all geared up, I usually lay my play mat out in the centre of my living room and go to town with my favourite baby toys i.e. my plushies, my rattle, my soft blocks and my stacking rings. When I get bored of playing I'll usually throw on some cartoons or an animated movie and I'll sit and watch that for a while.
Around the late afternoon 2pm or 3pm, I'll cuddle up with my plushy and blanket and I'll have a nap. Following my nap, I like having a nice warm bottle of milk. If my diaper is well-used, I'll change myself, diap up again and I'll usually play with my toys some more. Sometimes I'll vary things up a bit, for instance, if I have any baby food or cheerios handy, I'll throw on my bib and have a snack. When I'm in slightly older baby/toddler mode, I often feel like exploring, so I'll crawl around my apartment, sometimes it's just fun to see things from a little perspective.
Before bed I'll usually check on the condition of my diaper, if it needs a change I'll change myself and wash off in the bath, bath time can be fun. After that, I usually throw on another onesie, romper or a PJ set, I'll play some lullabies from my laptop and speaker system and when I feel decently dozy, I'll turn off my laptop, snuggle my plushy and go to sleep. If I don't have any obligations the following day, I'll repeat this routine, but not necessarily in the same order.
If you have a partner, a mommy/daddy, a switch or a fellow AB, your baby day routine will differ considerably. I have never had the luxury of experiencing a baby day with a caregiver, but I have had playdates, those can be very very fun.
Hopefully this gives you some ideas for your impending baby day. Lots of other AB's have given you some great suggestions as well.