You could book a flight for late at night, use it to pass security earlier in the day, for example because you would like to go standby on an earlier flight. If your plans for flying change after you have seen the exhibit, you can always exit the airport secure area, cancel the check-in and then cancel the award online to get the miles back. To minimize complications, I think that canceling while the check-in for the flight is still open would be recommended (let's say no later than one hour before departure to be on the safe side).

I experienced similar circumstances a couple of years ago, first taking my parents to the gate (to make sure paper upgrades on a LH flight were handled correctly), and then changing my mind regarding boarding a later flight I had a refundable ticket and got the money back on the credit card within a day.

PS: if you have a reward flight, to get the miles back online you have to cancel the reservation first, and then request the redeposit of the miles (there is a link for this in the canceled reservation). This way my redeposits have been always instantaneous. Calling carries the risk that the agent only does step one, so miles remain in a limbo...