OP - what is your budget?
You could give it the full road bike treatment: drop bars, brifters, road crankset, tires for around $400- using used and/or low end components.
The cheapest is new road tires and tubes - which will run you $50- to $150- depending on which you pick. This is the MOST effective change.
If you are spinning out in top gear, a road crankset would help; low end $50 - plus likely new BB ($20-) and probably new FD ($20-).