How to customize hover information in ggplotly object?

A cleaner way is to just add everything within the ggplot environment, using aesthetics two times, in order to pass just the full, single object onto ggplotly():

p <- ggplot(mtcars, aes(label = gear, label2 = hp)) + 
     geom_point(aes(x = disp, y= am, color = as.factor(cyl)))

ggplotly(p)

You can include required variables in aes() then use tooltip to specify which should be displayed:

p <- ggplot(mtcars, aes(x = disp, y= am, color = as.factor(cyl), 
                        gear=gear, hp=hp))+geom_point()
ggplotly(p,tooltip = c("x", "gear", "hp"))