For your highchair issue...the one we have is adjustable so I set so it can be pushed up to the table. You can get one of the boosters that strap to a chair, they can also be strapped in, they are great my mom has one for dd.
Stamps are fun...there are washable inks, they are messy but the ink washes off pretty easily. Dd likes to rub the ink on her arms
A chalkoard is fun and the chalk washes off easily. Dd drew many murils on our walls and ds would go around cleaning it off with a sponge. Sidewalk chalk is great.
Glue sticks...not sure if they are toxic
we have little foam shapes, they glue them to paper or themselves.
Magnadoodle...not messy but fun.
My dd is the messiest messy player ever, she also loves to paint and color her body so I make sure I only let her have the messy stuff when I have enough time to throw her in for a quick bath.