The Infinities are a nice choice. They take a bit of working with to fit in there properly cuz they're kind of deep. MTX makes an alright coaxial driver with a swivel tweeter as well. The Pioneers that were linked are a decent sounding coaxial too and on for cheap.

With a budget in the $250 range you can probably score a component set with separate tweeter and midrange. Generally better sounding than most coaxials.