Best way I ever found to pad out a post without it being too much meaningless filler is to add sensory information and body language to the usual "reactive writing" that RPing usually involves. And sensory goes beyond the more obvious things--we often describe whatever room we've entered, or where we are standing in it relative to other people. We often describe how our characters see the actions of others. But what does your character SEE? "Bob entered the banquet hall" vs "Bob entered the banquet hall, his eyes falling on elaborately embossed silver plates and goblets with long, delicate stems. The tableware glittered in the light from two high, stained glass windows, throwing splashes of red and yellow, blue and green over the dull, ancient stone walls. The bricks had been scrubbed so many times that the mortar wore thin, but from floor to ceiling there was not a speck of dirt nor moss."

This establishes an atmosphere as much as it does a visual continuity--if you'd described the cups and plates as "dented tins with rusty handles" and "scratched and cracked wooden bowls" and the walls as "cracked and peeling, with a creeping vine sneaking over the windowsill" you would have a very different MENTAL image of that castle--who owns it, its history, etc--as well as a physical image.

Smells and sounds often go ignored entirely in RPs in my personal experience but invoke the most internal associations because, unlike our eyes, they are not a direct 1 to 1 translation of our environment. A "crash" might be the sound of two cars coming together, or it might be the sound of someone falling down stairs. A "cloying, sour smog" might come from rotten fruit or noxious gas. If the character doesn't know, they have to speculate. Strong sounds and smells also evoke physical and emotional reactions--we recoil or flinch, we wrinkle our noses or cough and gag. Those actions--or, in some cases, lack thereof--can show a character's likes and dislikes, or give them information that influences their thoughts. Gorou figured out that Rhea was a dragon rider while several others were knights because he could smell horse on them, and "something that was definitely an animal, but not a horse" on Rhea.

The feel of touch is mostly only good for temperature--though that can still affect the atmosphere--and describing what goes along with physical action, so it's less useful for setting a scene. Great for fights though--when swords clash hard enough to shake bone and numb flesh, you know stuff's hitting the fan.

Body language is great because it speaks to one of the oldest, most often repeated pieces of advice given to writers: Show, Don't Tell. I also like body language because you can pull some whiplash with it due to the fact that different people have different "tells," and different emotions can sometimes be shown in the same way--some people shake because they're scared, other people shake because they're so angry they can't hold it in anymore. Here are some examples:

"Where are you going?" John asked nervously.

"To a friend's house." Susie said nonchalantly. "Why?" she asked quizzically.

Both of these suck because they state outright how a character is speaking or feeling, but they are telling, not showing.

"Where are you going?" John wrung his hands together.

"To a friend's house." Susie tilted her head and blinked. "Why?"

These use action to show how they feel. From these you can infer that John is nervous but Susie isn't and is wondering what's up with John. If you left it here, the reader probably assumes that John is a timid, maybe nerdy kind of guy. From the context, maybe he's trying to strike up conversation with Susie or maybe he's trying to come up with an idea that will make her want to stay and hang out with him, cause he's got a crush on her. Now, remember that whiplash I mentioned?

"Where are you going?" John wrung his hands together.

"To a friend's house." Susie tilted her head and blinked. "Why?"

John reached for the knife.

Now how much did your impression of John, and what he was feeling in the first sentence, change after the last sentence? Learning to write body language--and read it in other characters--helps a lot in figuring out what characters are feeling when they speak or do anything else, and can add a lot to a post even if all they're doing is talking to someone else--because between sentences, they're going to hop from one foot to the other, or they're going to rub their chin, or they're going to smirk and laugh.