The trampoline sounds like it could be a real problem. On the other hand, maybe if you have a thick rug or carpeting it won't be too bad. It'll also depend on the construction of the building, so it's hard to say. The only way to really know is use it and poll your downstairs neighbors about it. They will most likely appreciate your concern and you can relax knowing that you are not disturbing anyone when you use it (if this ends up being the case).

RE: the waterbed: many apartments forbid them. I would check your lease before installing it.

There are exercise mats made specifically to put underneath exercise machines in order to control vibration and noise. Check online and you will find them. I have not seen any for trampolines, but maybe there are some all-purose ones. Search for gym flooring, mats, noise reduction (etc).

I agree with above poster's suggestion. Also, you may end up with a downstairs neighbor who is out of the house certain hours predictably (you will have to ask) and you could do your jumping during those hours.

The trampline just sound bad. Let your neighbors know what the noise is coming from and when you plan to use it. Perahps they will be gone and its no problem.

Do check your lease on the waterbed. I don't allow them in upstairs apartments. The leaking is a minor issue compared to the structural damage they can cause in a stick built building.

Do you have access to a basement? I've seen those things being used in laundry rooms. They are lightweight and can be carried down to the basement or stored in your storage locker.

Sure, it will be a little extra effort to exercise in the basement, but it may be worth it to keep peace with your neighbors.

I don't know any place that allows waterbeds anymore these days.

I never even thought of waterbeds being restricted but it makes sense. Thankfully we got it free from someone who didn't want it anymore. Sounds like way too much trouble and worry even if allowed - I'll just use some of the wood for scrap :)

I will check online for your suggestion on a mat, never heard of that either. Not sure if it would slide if I put it on, say, a sofa cushion not used? SUrely vibrations couldn't be felt, heard through that? Just don't think that would work well with something I'm jumping on. :) May just be forced to use it outside, will have to see. And no, no basement in the building.

Thanks guys!

I would think a good cushion under the trampoline would work fine.

As for the waterbed, there are liners that you can get (and should, actually) that sit between the mattress and waterbed frame. This is designed specifically to deal with a mattress that springs a leak. I do not believe they are overly expensive.

I appreciate everyones help on this and wanted to follow-up. As an update, I am now lucky enough to have nabbed a grond floor apartment! I dont have to worry so much now about the people below me :) Still ditching the waterbed, of course, but the trampoline should now be fine. Thanks for all the advice, everyone!