The OP asks about baby and diaper play when it comes to roleplay activities, but as I'm a little leaning towards middle (9-12) or a middle (teenage 13-17) in most scenarios, I can really only answer to that.
When it come to the former, my favourite thing is going to the doctor (see several of my posts in medfet and a couple in age play) although there are others that would work well for me. Going to the doctor is the scenario where I get a boner as a kid, hence I like playing those memories again as an adult and fulfilling fantasies of exams I never had, but being a ‘big kid’ for me is also about non-sexual age play as part of my kink-life, so there are other situations I like revising just to feel like a kid again. Going to the dentist at that age is a good one for me, as is various activities I remember from school, such as doing different types of homework. As if to 'cross-reference' them, I do remember having a project at school involving the human body where we had to do a lot about human biology, and doctor-related stuff came up. I remember one worksheet about the heart and lungs where there was a picture of a teenage kid being examined with a stethoscope that got me excited, and a worksheet where one of the instructions was ‘Put your ear to a friend’s chest and listen to their hearbeat. What do you hear? Can you hear their valves opening and shutting?' I was too bashful to do that when I was actually that kid, but reliving that and being the kid who could do that as a role-play thing is a major pleasure for me. I really like imagining I'm back at school doing all that again and I'm as confident about it as I've become as an adult who doesn't mind talking about this now. I even wish I could involve my teacher and a friend I remember where she says ‘Go on then, put your ear against Alex’s chest and listen' and then I do the same to him. (Sadly I heard that the friend I'm thinking of passed away a few years ago; I think it was probably good I never told him he was my first boyhood crush on a guy and that I wished I could have done that to him; not to mention he had asthma and I was always secretly jealous that he got more check-ups at the doctor than I did because of it.)
I'd also quite like to do colouring as a younger kid, or maybe year 3-4 homework, in a non-sexual/non-medical context, with a parent enforcing the rules but praising me because I was actually kind of smart (I was smart in real life, but easily distracted and a trouble maker who couldn't ‘understand that he’s ever in the wrong' - and that really is a direct quote from my year 4 school report, aged 8-9)
As a middle there's one thing I love more than anything: video games. I'm not a 'gamer' nowadays, because that whole fanbase seem pretty hardcore and serious about modern gaming, but as a middle in real life I loved playing Sonic the Hedgehog or Street Fighter II or loads of other games rather than doing my homework or doing a serious hobby that might further my life/career/CV. I'd love to play that with another middle there encouraging me doing it. There are loads of fond memories I have with one particular friend of mine where we gamed ourselves silly on the original Playstation 1, and even before that my friends and I loved the Sega Megadrive and the SNES (my friend with asthma had the latter, and I do remember gaming with him some days after school and his mom would come in mid-game and ask him to pause it so he could take his inhaler…stopping an activity to take meds was something I remember being told to do as a kid too)
I'm a ‘big kid’ in real life because of all of this combined. I have gaming memorabilia and also ‘nowadays’ stuff that you can buy from gaming shops, eg plush toys of video games characters. I also have a stethoscope I bought myself and I love being a curious kid with it, trying it out on myself. I also still play the games my friends and I used to play, and enjoy taking a trip down memory lane by looking up some of the stuff I did at my primary school online (years 3, 4, 5 and 6 especially - for those who are non-UK members that's age 7-11) and seeing who else online reminisces and enjoys fond memories of that…and perhaps also got a school medical or was off school for a while as a sick kid with memories of going to the doctor.