As we've gone through speech therapy for my daughter with Down syndrome, we've been working with therapists who teach early childhood development.
Gross motor skills come before speech, which is why babies can sign before they can speak. Baby signs that are useful include: more, all done.
At my 2 my daughter with Down syndrome knows: want, all done, more, book, water, bath, shoes, socks, nose, head, sleep, rice (her favorite food), banana
Other signs she doesn't use initiate as much but can recognize include: it's time to play parachute, bib.
Signs that really motivate them like food, or ways to ask for control are the easiest to teach.