Last week we had an entire game of DOOM in a QR code... I think someone can get us "Do nothing" in less than 300 lines of code.
<input id="i">
<button onclick="l.textContent='Now do '+i.value">Do This</button>
<button onclick="l.textContent='Do nothing'">Do Nothing</button>
<label id="l"></label> data:text/html;,<input id=i><button onclick="l.textContent='Now do '+i.value">Do</button><button onclick="l.textContent='Do nothing'">Nope</button><label id=l>