A few easy to pull off options...
1. Tighten the screw on the blockade of neutrals/non-belligerents: either have Britain demand certain concessions to crack down on trans-shipping (total oversight of customs, observers on domestic merchant fleets and fishing vessels, ect.) to allow imports of ANYTHING that could be potential war-material for Germany, or at least reduce the "ration" they're allowed for domestic consumption to much lower levels. While the Blockade as an act of war certainly annoys some businessmen, flagrent disreguard for the rights of neutrals would have a far greater effect, especially if it starts producing starvation in Scandinavia
2. Unilaterally (I.E not in line with Germany) prevent American relief for Belgium through on the suspicion/assertion that the Germans will just confiscate the food. Its a pretty widely-supported humanitarian project in the States, and it makes the British and French look kind of hypocritical after harping on about all the hardships Belgium is being forced to endure.
3. Support with troops and weapons Koronov's military coup in Russia, especially if its done preemptively when the Czar's regime is starting to look shakey, for the sake of insuring Russia remains in the war. Nicky's government was seen as the poster child for everything wrong with autocracy, and working so hard to save it would really rub liberty-loving America the wrong way.
These align well with existing war-policy and aren't extreme enough to run the risk of war/embargo from the U.S, which isen't something the Entente governments would be particularly inclined to see happen.