You could also try to repair all the bridges from the east coast to the west coast, but I'm not sure you'd cross the Appalachian trail with that little of a budget.
Which, for instance?
If we're talking about megaprojects, a mini-Manhattan Project of sorts for developing better theories of superconductivity would be extremely beneficial. Practical room temperature superconductors would totally revolutionize our society. We're close, but we're not quite there yet.
The usual question is what to do with the those who don't know anything about that, don't really want to. Should particle physics simply ended? Should they go and beg for money from billionaires?
We don't coddle other professions with declining ROI like that. Why should particle physics be an exception? Anyone else would be told to "learn to code".
Except when folks accidentally elect someone who keeps telling coal miners that let's bring back coal.