The Airtrian does not go to LGA - only JFK. Nor does the subway.

From LGA your options into the city are:

Taxi - about $25 -$30 (depending on where you're staying - more if very far downtown - vs uptown or midtown) plus tip and bridge toll - holds up to 4 people - easiest and fastest.

Shuttle bus - for drop off at a series of hotels. Less expensive for 1 person - for 2 or more not really less than a cab.

Pubic transit. Involves either taking a city bus into Manhattan and then changing for another bus or subway to your desination. Or taking a Queens bus to one of the local subway lines into Manhattan - then changing for one to get to your hotel. The cheapest - but very time consuming. And neither bus nor subway has anyplace for you to put luggage - so not recommended if you have more than a small carry-on.

I just got back from a weekend in NYC but we stayed in Secaucus and took the bus in every day. The bus was fantastic. Hotels around the area of the bus stops were Courtyard Marriott, Hampton Inn, Amerisuites, Holiday Inn. Hotels are cheaper, no cost to park your car and you leave the driving to someone else. $2.60 each way. Bus runs till at least midnight..1am on weekends. It is the way to go money wise and hassle wise.

Did you try Days Inn on 8th ave? Decent hotel, easy to drive to and parking garage next door. There is a fee for parking but we added it to the cost of the hotel. Also, this hotels gives discounts such as AAA.