Hi Kimberly!
I think my biggest thing is only putting out what I’m comfortable with; as you said, its no fun for either of us if I feel the need to hover! I did often sit with my own “project” at first so I could offer gentle corrections, as needed. There was a lot of reminding at first with the mat, and if she moved the supplies I would quickly place them back (being positive but firm) — by making the boundary really clear but giving her freedom within that boundary, she stopped testing and starting exploring how much freedom she truly had.
I felt comfortable with nontoxic crayons, washable markers, and homemade finger paint when she was that young. We make homemade play dough so its safe to taste, but even that can get into carpet at first. You could also tape down some contact sticky paper and have him attach pompoms, feathers, etc?
The teacups are somewhat sturdy, but I’m a big fan of the Montessori principle of giving children delicate objects to encourage them to be confident and gentle. (But even her few plastic toys, I’m really insistent that only balls get thrown, etc.) I notice how gentle my daughter is now as a toddler with her toys and materials, and I think its because she feels that her materials are special and hasn’t had the experience of throwing a plastic cup and watching it stay intact — there’s an intrinsic motivation to be gentle and keep the teacups. If she ever did break one, esp if it was due to misuse, I would wait a week before giving a new one… I might also put the crayons for that cup away for the week. I try to only put out what I hope she can handle, but nothing that I’d be devastated if she broke.
I hope I answered all of your questions, but please let me know if you have any more!