Real Estate Law
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You can be found liable for injuries to third parties on this trampoline and if someone was to get hurt, you most certainly would be sued and even if you weren't found liable, this would be a costly proposition. Your lease likely does not contain any language which would prohibit trampolines, otherwise you would not be in this position. So my first piece of advice is to prohibit trampolines in any future leases.
But what do you do with this trampoline? My advice would be to talk to them first and ask them to remove it. Tell them you fear liability and tell them you are aware some homeowners insurers will cancel policies over trampolines, which are considered "attractive nuisances." If they do not agree to remove it then I would advise you to send a letter demanding removal by a certain date. Then, if they still fail to remove it, your only option is to go to court, which obviously everyone would prefer to avoid.
Although not California specific, here is an interesting forum thread on this topic you should review http://absoluteca.websitetoolbox.com/post/discovered-tenant-has-trampoline-2894628?&trail=15
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