I has a surprisingly fun time with a T-Rex. The character of this comedy was a 13th level fighter (swashbuckler type), who through a failed save verses a spell, found himself randomly teleported somewhere in the Realms, namely, Chult.
Said fighter started trying to find out where he was and stumbled into a chance meeting with a large T-Rex. Rex wins initative and snatches up the fighter and a two failed grapple checks later, finds out first hand what a T-Rex stomach looks like from the inside. On his action, the fighter manages to cut himself out with a ordinary dagger. Which is fine, except it takes his entire action to do it and he is last in initative.
The dumb T-Rex, is delighted to see more food (never drawing the conclusion that he just swallowed this creature earlier). So what happened next was a pattern which repeated itself for some 5 - 6 rounds. The T-Rex would chow down, the hapless fighter would fail all grapple checks and would get swallowed, only to cut himself out. Lather, Rinse, Repeat. In the meantime, all the other players were literally in tears due to the comedy of the situation and the growing frustration of the swashbuckler's player.
It was a race to see who would permanently check out first - the swashbucker or the T-Rex. The swashbuckler ended up with 3 hps left when the T-Rex died.
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Mimics are a hoot too.....especially advanced ones. I had an advanced mimic literally 'jelly roll' the entire party once, as the mimic rolled up on itself and trapped the party within the 'rolls' of it's flesh. What a fun battle that was. The battle left such a impression that my players refer to actual jelly rolls brought for snacks as 'killer mimics' during our play sessions.
