You probably want the ability to collar their stock at very least and depending on what you think of their prospects you may ask them for a ratchet in the case of a stock decline. They probably won't give you the latter.
You want triggered acceleration of that vesting if they terminate you without cause. This is sometimes called double trigger vesting (first trigger is acquisition, second trigger is termination). You also want a decent employment contract in the meantime.
They may in fact be in position to say "this is the offer, we don't go back and forth on that" but what that USUALLY means is "this is the PRICE" and the terms thereof do get negotiated.
You need to make sure that you can actually DO those million ideas you have for the future. If these guys seriously acquire you and kick you out in three months, or acquire you and assign you to do something other than build your existing biz, you are probably going to be super disgruntled. An awful lot of acquisitions turn into "ok, that was nice, now do this instead." And hell, they might be buying you JUST to shut down a competitor. Heaven knows that has happened enough.
If you have good angel/VC contacts, this might be an ideal time to say "hey I have an offer, I'm not ready to give up at this point, give me a terms sheet for growth capital on these terms" and run like hell. Keep in mind that this is ALSO likely to be shark infested waters.