MHK->ORD->LGA->Central Park
HINT: A transfer from the bus if you pay with coins can NOT be used on the subway; it can only be used on another bus. Note that if you use one of these transfers obtained on the bus, the slip actually reads "Bus Transfer."
MetroCard solves this problem!
At LGA, go to a Hudson News stand within the airport. Get a $4 MetroCard. Use that card on the bus, and then use it again to get on the subway. The free transfer will count, and you'll have $2 remaining on it.
As you board the subway in Astoria, find a MetroCard vending machine to either refill your current card, or buy a new one. For maximum value, buy at least $7 worth of card, and it will add a 15% bonus. For example, paying $20 will give you a $23 card. $40 will give you a $46 card. Anything less than $40 is annoying because you'll have that pesky $1 balance left unless you use a $5 express bus along the way. Nice way for the MTA to get people to not bother using their whole cards, and make a little extra cash off of it.
You can also buy an unlimited-ride MetroCard; one-day, seven-day, or 30-day. These cannot be used on express buses, but that likely won't matter for you; it's still a great idea if you'll be using mass transit to any significant degree.
If you're going right back to LGA without doing anything else in New York, then don't bother with the new card.