The following is a set of features for the more obvious of trapdoors set in ceiling and floors.
Trapdoors are not always easy to open.
Trapdoor Opening Table (roll 1d100)
1-20 Locked: must unlock trapdoor to open.
21-30 Sealed: the trapdoor is sealed shut with spikes and or boards. It will take 2-12 rounds to force open.
31-35 Wizard Locked: the trapdoor is magically sealed.
36-70 Stuck: the trapdoor is stuck and requires effort to open. Use standard rules for opening doors.
71-80 Heavy: the trapdoor is particularly heavy and requires a combined STR of 15+ 3d10 to open.
81-100 Free: the trapdoor will open freely.
Trapdoors will be trapped beyond the features listed here 15% of the time.
Trapdoors (roll 1d100)
1-10 Pitfall: Surprise surprise the Trapdoor is over a pitfall, adventurers tampering with it will fall in on a roll of 1-2 in 6. (re-roll if this trapdoor is in the ceiling)
11-25 Pest Hole: a nest of monsters dwells in a small chamber beyond the trapdoor.
26-60 Threshold: the trapdoor leads to a room/chamber of the same size as the current one (or 30feet by 30 feet if found in a corridor), this room is otherwise as ordinary as other rooms in the dungeon.
61-70 Gateway: This trapdoor connects to another level (or sub level)
71-85 Treasure Cache: a nook beyond the trapdoor contains a treasure. Treasure may be trapped.
86-90 Misty Barrier: opening this trapdoor causes a mist to billow forth. (see Mist table). Examining the trapdoor carefully could reveal a small amount of the mist (1 chance in 6), that will not harm or benefit anyone but serve to forewarn dungeoneers.
91-100 Dungeoneer's Cache: other adventurers have stowed supplies here in the past. See Dungeoneer Cache post.