I was going to suggest the Redeye Grill, on
7th in the 50s. It's casual, quite good, lots of fun,
but somewhat expensive.
Walk further west. 9th and 10th Avenues have dozens of
inexpensive restaurants. Try Mangia e Bevi, which I
believe is on 10th in the upper 50s. Simple,
unpretentious Italian at good prices (or at least it
used to be; I haven't been there in years).