Changing things up means more than just the melody. You can have 16 melodies if you want - but if they're over the same bed of other sounds, and the same bassline, and the same drum beat, then it's not going to sound all that different.
Plenty of beats feature a similar theme throughout the verses and hooks, but generally the hooks will be fuller (more full?). Start with your hook, and then create the verses by cutting elements. By the time you get back to your hook, there should be 2, 3 maybe 4 elements the listener has NOT heard in the verse that beefs up your chorus.
Taking it a step further, you can add a bridge, where the chord
progression can change up completely. Just make sure it makes sense with the rest of the song. Going even further would be to make your verse and hook 2 completely different progressions/melodies/everything. I like that style, but I actually don't hear it too much in commercial rap songs.
Anytime you change parts, try to use some kind of transitional effect - an uplifter, sweep, reverse cymbal, etc. - and get some kind of "hit" on the 1 of the new part, like a crash cymbal or other effect/noise. These are things I do to spice up the track and aid the listener in realizing there's a change.