I Intend to have my players discover the completely devastated site of an old mages tower, blown up by a magic accident. As a result of the accident, the veil to the Astral plane is especially thin in that place and arcane magic is strengthened extremely.
The details of the story don't really matter though, what this is about, are the mechanical effects.
I plan on giving all arcane and SLA casters the option of "tapping into the energy" of that place, thereby doubling their effective caster level, in exchange for haviing to make a pretty difficult caster level check to keep the magic under control. (if they don't make it, out comes the random table) I don't plan on having my PCs fight in the ruins, it's just supposed to be curiosity for the braniac Wizard and a possible trading good for negotiations with another wizard that will turn up later in the story line. Now I want to make sure my players don't surprise me with an unexpected exploit of this. They are level 9 right now, but quickly rising, so feel free to warn me about higher level spells too.

What I want to know from you is: What would you do with double caster level? What ways come to your mind to exploit this in non-combat situations? I want to be prepared for what my players might throw at me.

Hmm, I'm not answering your original question, but why are you "giving casters the option" of tapping into this energy. If the veil to the Astral plane is thin, then it should be all the time sort of thing. Also, this seems like it would at least partly balance out the possible exploits you're worried about, since your players can't choose to turn it off to cast mundane spells or spells they don't want increased.

out of combat?

If they have permanency, this would be the perfect place to use it. They can make their permanency almost impossible to dispel. Not much other than that. There are a ton of in combat exploits possible, but most spells wear off within a day or so. If any of them have magic crafting, they could use it here for super high caster level gear.

Also, I would want to set up traps here in the future. I would try to lure back enemies to this place so that I could fight them here.

What about the primal magic rules in Inner Sea Magic. This looks like a great place to use them.

Enchanting things they normally could not enchant such as +5 weapons, armor and cloaks of resistance at level 8.

The aforementioned Permanence spell.

Scrying on people with protection less than mind blank (need a level check to beat there non detection no problem).

This would be an ideal place to get a permanent magic fang.

http://www.d20pfsrd.com/magic/all-spells/m/magic-fang

@ Mathobedis: the primal magic rules (slightly altered) are already incorporated on a failed CL-check, I got that part covered. I Just want to make sure they don't break my campaign by using this place for something else.
The dispel-save permamency might be useful for them, but since they tend to be VERY optimize-lazy, I'm not afraid of that. I had already thought of the super-scrying, I tend to protect my plots against this anyway, if I don't want them to scry on stuff. (They have a crystal ball, so this happens quite a lot) The super-crafting will be no problem, since they simply don't have the money for stuff like that. They are a pretty selfish bunch (IT) and so far didn't get the idea of pooling the party ressources for some big bad magic item.
So far it looks like I should be OK. :)

Mechanically, I would only have the area increase caster level by say +5, not double. At 10th level, they would be casting spells at near demi-god levels (20th), not just at a "heightened" level.

My other thoughts and concerns are mirrored above in previous posts.

I agree with Mahtobedis. This would be a great place for primal magic.

Enchanting things they normally could not enchant such as +5 weapons, armor and cloaks of resistance at level 8.

You can do this already by adding +5 to the Spellcraft DC (excluding constructs).

@RD: As stated above, the primal magic rules are already in place.
@Swellsword: I know doubling CL is immensely powerful. I WANT the possibilities of this place to be godlike, because I want to tempt the players into being careless and experimenting. The group fought a lot of SR-heavy enemys lately, so giving the casters a little "God-time" is intended, since especially the sorcerer rolled really low a lot when trying to overcome these SRs, so the character as well as the player are a bit frustrated.
Depending on what they intend to do with the place, I might have some monsters/other casters come in and try to snatch it from them, because they have already made 2 different enemies who keep tabs on them without their knowledge. Therefore, making it so very powerful is a good thing again, since it increases the interest of the "bad" guys and the fear of loosing it for the PCs. So I think I'll stick with the doubling.

Thank you all for your Ideas, anything else? Because if that's it, I think I can cope with that. :-)

If I was the Wiz with 5th level spells I'd be using Wall of Stone to build a base (and Teleport to remove at least one leg of any journey).