Be careful, as it is easy to teach them things they need to unlearn later! Teach them to write lower case letters rather than capitals, first. Make sure they are made the cursive way, not ball and stick, for example.
Key at this age though is language and motor skills.
Motor skills: Trampolining, climbing, scooting, play dough, bead threading and getting dressed etc are all really important to posture and grip when writing. Read some articles by physiotherapists about the importance of early years development.
For maths, it's also language. Ordering by size, height etc and appropriate language. Ordering by number and appropriate language. Position. So using words like more, less, first, last, bigger biggest, taller, shortest, heavier, on top, underneath, next to, beside. Make arrays of items, look for and complete patterns.
Social skills also need to be learned at the correct age, so make sure DC is mixing with lots of children, playing games etc.
There are some concepts they simply cannot get at this age. You can teach them to perform it, but they don't actually grasp it.
Most importantly, to love books and follow their interests. Go to interesting places, use a varied vocabulary, and play play play!