Reviewer wants me to do x - Should I admit I don't have the resources to do so although I think it's a good idea?
When I have been faced with a situation like this in the past, the course of action that my co-authors and I found best was to get in touch with the handling editor to negotiate.

Editors are the ones who will generally make the call, and if doing the experiments will pose an high burden in the time-frame available, it's worth having a discussion before starting them.  Sometimes a reviewer has pointed out a real flaw and more experiments really are required in order to deal with it.  Other times, a reviewer is just giving "wouldn't it be nice if..." homework.  It sounds like you think it's more the latter, but the editor may disagree.  I would recommend starting with an explanation of why you think your current experiments are sufficient for a significant publication and why the new experiments should be pushed into future work instead.  The discussion with the editor is likely to end up in one of three places:


The editor agrees that the experiments aren't necessary, and you can talk about them as a direction for future work.
The editor decides your paper will be rejected, since you can't do the experiments.
The editor agrees to a "middle ground" where certain critical studies are done (it may surprise you which ones), perhaps with an extension on time to allow you time to do them, and the paper moves forward.


Talking with an editor is taking an additional risk, since your paper might get rejected quickly, but I would recommend a "fail-fast" strategy over putting in lots of time and effort that turns out to be futile because you were working on something that wouldn't be enough to satisfy the editor in any case.