Congrats on the new relationship! And both sharing an interest in ABDL, while a relationship definitely has multiple facets, that'll definitely be a great and fun common interest to share and explore
Taking things slow is definitely the safer and right call imo. I've seen issues caused by taking things too fast.
Of course every person and relationship and interests are different so it can vary, but personally one thing I enjoy doing with my ABDL friends is the "typical friend activities", like going to movies, getting food, watching shows and gaming...except while diapered, in cubby clothes, and with plushies (at least when staying in). Watching Bluey and other cartoons and animated films is a big plus and something that I can't normally do with non ABDL friends. These sort of activities are fun to do with anyone, but being able to do them while in little mode and getting to mix little stuff in with it does make them even funner and more special. Not that I do it as often, but going to the zoo and hiking is also super fun, as long as you don't have diaper chafing issues!
Cuddling while all padded up is super fun too! Being close with someone you like is always nice and assuming you're both comfortable with it, being able to get and give reassuring diaper pats and rubs to a friend's diaper while cuddling (or just hanging out in general) feels super validating and good :3
While you take things slow you can learn each others' boundaries and what you're both okay and comfortable with and as you get more comfortable with each other you can always try new things.
One thing that's kind of unique to ABDL relationships and that I've noticed is to a lot of people in ABDL relationships, I don't want to call them "rules" exactly, but allow a certain level or "type" of interaction with other ABDLs. This totally may or may not apply to the both of you, but I have ABDL friends that are in relationships and some of them are okay with giving and/or getting diaper changes from others and being diapered (in just diapers) around others. Since giving and getting diaper changes is a relatively intimate thing, but can also be a pretty common and big part of being ABDL and can, but not always, have some aspects of kink/fetish/sexuality mixed in with it, defining boundaries of what type of activities you do with other ABDLs might not be a bad idea.
The main thing is though, just have fun and enjoy being a part of each other's lives