For horizontal boxes AABB is great, for slopes, you need to add a twist to it.
The height of a slope that's positioned between x1 and x2 at a specific point can be calculated the following way (x is the point we want to get the height at, h1 is the height at x1 and h2 is the hight at x2):
height := (x - x1) / (x2 - x1) * (h2 - h1) + h1
Now, first you need to check if the player collides with the aabb of the slope (this is the broad phase), if it does, then get the height of it at the player's position.
Next, check if the player's height is less than the result, if it is and he's fallong, then set his y coordinate to the height of the slope and if he has one, set his vertical velocity to 0.
This doesn't work with the underside of the slopes, if you want that too, you need to do the same for that too.