How can charity monetize parking lot?

I think you have misstated your question. The question is not how to maximize the use of the parking lot, but how to maximize the net revenue from the use. You could probably maximize the use by letting anyone park on your lot at any time for free, but that wouldn’t generate the revenue you want.

Assuming your charitable exempt purpose is not to provide parking for the public, you are probably dealing with unrelated business taxable income and unrelated business income tax (“UBIT”).  If you own your parking lot unencumbered and you leased it to the gas station owner to operate, you might be able to avoid UBIT. But a lease is normally for a specified period of time, longer than the length of a baseball game.  The IRS would undoubtedly argue that this type of income came from licensing use of the property for short periods and declare the income unrelated business taxable income.

Unrelated business taxable income is not illegal and won’t jeopardize your federal tax-exempt status unless it is “too much” and more than an insubstantial part of your operations.  (See Ready Reference Page: “Nonprofits Often Worry About UBIT”) (If you were a 501(c)(7) social club you could lose your exemption if you had too much revenue from non-members.)

You might maximize your net income if you run the operation yourself.  (You might even avoid UBIT if all of the attendants are volunteers.)  You might get as much money with a lot less work if you allow the gas station manager to operate the lot for a percentage of the gross, not net, revenue.  (The IRS doesn’t like arrangements based on net income.). You wouldn’t be able to classify the gas station owner’s payment to you as a contribution because it is obviously a quid pro quo payment that would not be made without the right to sell access to the property for parking.

If you do one of these deals, you want to be sure that your own clients, patrons, or staff who use the lot for your own purposes are not unreasonably disrupted in their use.  You will want to be sure that your insurance still protects you if one of the drivers breaks an ankle stepping in a pothole on your now-commercial lot or is assaulted at night because there isn’t enough light on the lot. To be legally compliant, you may need a special local license, or even a zoning variance, for a commercial parking lot. And you will want to be sure that you don’t lose a charitable real estate tax exemption (perhaps on a much larger parcel) because of the new non-charitable use.

Generating new revenue is a goal of most nonprofits. You just have to figure out whether it is worth the effort and the risks involved.