Help me plan a heist!

[Dormammu]

I want to give my PCs a treat and run a heist or possibly a jailbreak.

They are all 5-6th level. Here are some of the elements I want to include.

Opening Montage: (Probably using Clairvoyance) to show them just how hard this mission is going to be.

Monsters and Traps above the PC’s CR: It’s supposed to be a heist, not a dungeon crawl, the PC’s get the advantage of foreknowledge but the disadvantage that this is going to be really hard.

Something goes wrong: As in all heists some of the info has to be wrong, or the crew must find out they've been double crossed.

All ideas welcome.

[VoxRationis]

Well, most of the guards should be basic, "Spot is a cross-class skill" types, but you should have a kind of guard, immediately distinguishable on sight from the basic type, that has a high Spot check. This provides a challenge without being a complete TPK recipe.
I'm not sure whether you plan on them breaking in and sneaking, Garrett-style, or whether you plan on a "The target is throwing a gala tomorrow to celebrate..." type adventure, but in either case, remember that Disguise and Forgery are both things. Again, most of the guards should be vulnerable to this (flat-out negating player abilities, especially skills, tends to reduce fun), but they'll happily give reports to their superiors and so on, and eventually someone will notice that the documents are forged, should the players just blithely hand over their papers for inspection and not ask for them back.
If you're planning on using Clairvoyance, you probably have a wizard running around. Put some AMFs in places of high security to make sure that the wizard can't cast knock or other instant-win spells. Better yet, put the doors a marginal distance outside of the AMF, so they can still be enchanted but the wizard doesn't have room to cast anything on those doors. This also has the benefit of not having everything revealed with a few divinations, making nasty surprises possible.
On that same note, remember that lead-lined walls for important rooms are a good way to block divinations. If you're a little generous with the rules, you might rule that gold, which is only a bit lighter than lead, also blocks similar effects. This would also help answer the question "Why doesn't this guy block all the rooms?" with an answer of "Expense," though in truth lead can also be expensive in some societies.
Reward in-depth reconnaissance. Make the target seem impenetrable at first, but secretly put in weaknesses that can be exploited if the PCs do their homework. Figure out what bars the guards frequent and probe them for information. Climb up to the local church tower (you can volunteer as a bell-puller or something) and get some birds-eye-view reconnaissance. Scout around the perimeter. Have the "bad guy" have a young, impressionable daughter or son a PC can charm and convince to give a tour of the compound.
Find ways to block some tricks, especially spells, but not all tricks and not all the time for any one trick.

[iceman10058]

expanding on the above a little bit, also have some monsters "guarding" highly important areas, make them monsters that are more animal in nature and have cr's lower than the highest level npc by enough so it seems reasonable to have there. this will make it so the pc's can easily deal with it, but have to do so quickly and quietly as to not attract attention.

[Dormammu]

These are great guys! Keep 'em coming!

I think rather than using AMF I am just going to have lots of locks on each door. Sure knock gets you through a door or two but there are 15 doors!
Also thinking of putting this all underground. The final room with the maguffin is completely encased in solid rock. The jailers or guards have to disintegrate their way through then use wall of stone to plug up the hole or similar.

I think this is more of a "mission impossible" or "escape from super-prison" type mission and less of an "infiltrate the ball" social mission but I'm still just brainstorming!

[VoxRationis]

It's going to be a lot of trouble to do so, but have the guards' patrol pathways and rotation schedules mapped out. You know the players will try to time their advances "to just after one leaves and just before one arrives," so you should know if that's even a viable option.

[iceman10058]

i would say have a beholder patrol the halls, but that may end up bad for the party...

[Scarce]

To differentiate the heist scenario from normal dungeon crawls, give the players the map of the compound before they enter it, complete with some (but not all) of the guard postings, traps, locks, etc. Make sure that there are many different ways to enter the building, and many ways to proceed. Then sit back and watch them plan and enact their heist.

[SliceandDiceKid]

I'm totally doing this. Great idea with great input!

[BowStreetRunner]

Make them use something other than Knock spells.

If you put a silence effect covering all areas within line of effect of the door, you will make it difficult for a wizard to cast knock (which has a verbal component).
I find it difficult to believe that no magical locks have been created that would ward against Knock spells - so in my games I have added such locks. Overcoming them usually requires a Dispel attempt to suppress the lock's magical properties, followed by the knock spell. The locks come in varying DCs, with those harder to dispel costing the most. Of course, mundane Open Locks attempts still work.
Multi-layered defenses are common when protecting something extremely valuable. An adamantine door with a superior lock, alarm spell, glyph of warding and an energy drain trap will be an imposing barrier to get through, especially with the two stone golems guarding it! Sure, knock will open the lock - but as for the rest...
I love false doors. My favorite is animate object on an adamatine false door that is just sitting there with instructions to attack anyone who tries to cast knock on it. :smallbiggrin:
Don't just lock a door. Bar it. Or don't use a door at all but instead a command-word activated magical portal to a pocket dimension. Or a sliding stone wall with a complex mechanism to open it and keep it open.

[Dormammu]

Glad you guys liked the idea.

These are all wonderful. Thanks!