I believe any US budget surplus should go directly to our debt. The United States has far too much debt out, especially to the Chinese. If we have "extra money" we need to pay off those debts just as many Americans have to pay off their own personal debt. This country is better off when it has little debt held by other countries.
The current budget surplus in the United States should be used to form a department of food. By this is meant not a department of agriculture with its agricultural subsidies to only the largest of farms, but smaller community plots that can teach individuals how to grow and cook whole foods.
I think the US budget surplus can be spent in several good area. One would be in raising the bar for science and technology and education. More money could go to healthcare and poverty. NASA could also get better funding. With a budget surplus, you could also begin to pay down the defecit.
A surplus happens when revenues exceed expenditure. Since revenues come ultimately from the taxpayers, then any surpluses ought to be returned to the rightful owners which is of course the taxpayers. If the money isn't returned it could jeopardize the surplus as legislators might undertake some program which isn't entirely covered by the surplus.