So this last session was an interesting one, albeit one that got co-opted by the players to a degree in odd ways. That's fine, I rolled with the punches. Let me explain.
First, I got an email from a player saying, in effect, that he wanted to change characters. To be honest, I hadn't really given the idea much thought. On the one hand, I want everyone to have fun playing what they want. On the other, there was the party to consider. While an optimal party isn't necessary by any means, trading one of the only 2 defenders (the other's a phb cha pally) for an Assassin meant a significant decrease in the party's durability. Also, I was trying to avoid the "Adventure of the week" play style. Not because I don't like it or because I need to control everything, but because it doesn't match this specific campaign and this is what we all agreed on at the start. In the end I ruled that a player could change out their character once per tier if they describe what happened to the old character, describe why the new character came in, and complete a background on the new character. So we ended up with one less Swordmage and one new Assassin at the table.
Anyway, I knew the players would be performing a grand "plot device" style ritual (aka Skill Challenge) that needed to be performed in the middle of combat. I made odd rules to the ritual so that when combat broke out, it would lead to an interesting (I HOPED!) encounter. Namely, the PC performing the ritual had to start the ritual with full hit points, couldn't move from a raised pulpit like structure and had to retain at least 75% of her hit points throughout the casting of the ritual or it would fail. Also, each check in the ritual required a standard action. The player could choose not to use their standard to continue the ritual without losing it completely (effectively pausing it).
In play, I realized that this was far too restrictive on the player. She felt a little left out of the action until I ever so subtly hinted that she could pause the ritual and cast flaming sphere and effectively have a battle presence and still seriously screw with the enemy party. In addition, she needed one extra helper to scatter components around the ritual circle (a giant 8x8 square circle, mind you). I ruled that the player aiding could move anywhere in the circle but aiding required a minor action each turn. This was a decent trade off since he is a warlock and couldn't curse very well.
The original plan was to have a semi complex set of actions written down for the players to do during the battle but I ran out of time so it was a series of uninspiring Arcana checks instead. That was a little disappointing. But it happens.
The battle went well regardless. I was going to try out some terrain powers from DMG2, but the players got the best of me again! The rogue decided to steal the caustic materials and the Paladin decided to barricade the door using the tables they could have tipped over! Well, at least they were being creative! ^_^
The assassin did great damage wise but that was actually an issue. His inherent lack of AC and hit points combined with the fact that he made himself a giant target (he took out 3/4 of a monster's hp in one round) meant that he ended up getting the crap beat out of him. Still the Wizard made it through unharmed so I guess they did their jobs!