The real money is in entitlements. For social security, we should raise the retirement age by 2 months per year into the indefinite future. We should means-test social security so that people earning more than $200K don't collect it. We should index starting benefits to price inflation instead of wage inflation, and we should index annual increases to a more accurate measure of inflation than the CPI.
We can rein in the costs of Medicare/Medicaid by busting the AMA monopoly on medical licensing. This creates an artificial shortage of medical professionals, which drives the costs up for everyone. We should also allow the government to negotiate the prices that it pays for drugs. The government should impose universal standards for medical documents and fund digitization efforts, since a disgustingly large fraction of each dollar of medical care goes toward paperwork. We should tax health benefits as regular income, to break the link between employment and insurance. And we can nudge people toward high-deductible plans, so that people are protected from major medical expenses but feel the costs of smaller procedures. Also, the government should pay people to get semiannual checkups (and mammograms for women). While it may sound silly for the government to pay people to do something that benefits themselves anyway, this will save money in the long run by avoiding some complicated procedures that will cost the taxpayers money.
And we should raise taxes selectively, in the places it's least likely to cause harm. One good place to start would be to end the home mortgage interest deduction, which is a horrendous distortion of the market and detrimental to the economy as a whole. We could raise $130 billion a year by doing this. Or better yet, it could be part of an overall simplification to the tax code. We should also raise taxes a few percent on the top bracket, and raise estate taxes significantly for the wealthiest estates. As mentioned before, we can also raise revenue by taxing health benefits as income. Finally, we can increase the tax on gasoline.