I've done this by adding an "extra donation" optional fee for $5 with multiple quantity enabled. This way they can make a gift in $5 increments. We'll usually get a couple people that will give an extra $20 or so.
Something to think about is whether to include it on giving statements. Are they getting something for this, even though the cost it's optional, or is it an extra gift beyond that where no good or services were received? We've usually included this option where people are already paying for their own fees and this is an extra donation on top of that to help cover, for example, those getting a scholarship to attend the event (which we would usually enter manually). So, you may want to make an entry in contributions.