Teach him fundamentals of arithmetic in a fun way: one thing I did was to tell stories like - there were 5 monkeys playing on a tree, one ran away, how many monkeys were playing together after that? Teach him skip counting, which is the basis of multiplication - again in a fun way - using either manipulatives or stories. I also taught my son small poems to memorize (he had a very strong memory and loved to use it). I enrolled him in a group music class at 3 and a group piano music lesson at age 4. I agree with Thomas Percy - if your son is ready for private lessons, enroll him in Suzuki Violin by all means.
I remember that we also did group gymnastics and toddler soccer at that age. In daycare, he self advocated to go and spend part of his day with the older kids who were in TK at that time - I think that being with older kids stimulated him a lot.
