if your altima's newer than '06 or so it's gonna fall apart while the value turns to nothing
my suggestions are the tenth gen civic, third gen mazda3, eleventh gen corolla... maybe a nice example of an nb or nc miata if you're into drop tops... miatas are the easiest whips to mod too, you can do a turbo or an ls swap and still be within your $15k budget
and did i see someone in this thread recommending cheap used germans to a young breh on a shoestring budget?
edit: tagging thecoli's car guru
@Schadenfreude