Use the maximum HP possible from the dice instead of the average given (eg. 6d12 = 72 instead of 39), or at least a higher portion of the maximum quantity
Increase AC
Give it extra damage of a different type
Give non-lair monsters lair actions, and give monsters with lair actions an even stronger lair action they can use when below half-health.
Same with legendary actions
Look at older DnD editions and see if the monster or any similar monsters have extra abilities you can add
Edit: I should have specified that these are in ascending levels of difficulty for the DM, but are also more interesting