Hi
"Good" for who? for you or for your customers. Because in this industry, it is unfortunately, often one or the other.
Ideally, you should find something something that is good for both sides.
Additionally, you need to differentiate between repeat business travelers and private 'holiday' travelers. For a business traveler, the 100 $/Euro discount on the next trip is more relevant because they know they will use it (and it brings you repeat business). For private individuals, they prefer discounts which are relevant to their current booking.
So:
If your system allows it, start by asking (like most website do) if it's a business trip or pleasure. If business, the 100X on next order often works. If pleasure, the discount needs to be relevant to the current order.
As for the actual type of discount/loyalty: there are hundreds of things to take into account, most of them are about understanding the psychology behind the people making the purchase. Most people will compare prices, so an offer that refers to comparison prices can help ("If you find a cheaper price we will compare it"), a lot of people hesitate before finalizing the purchase (so a discount offer towards the final stages is wise - if you your tech allows it, then this should only be offered to users who 'pause' on a certain page and are hesitant to move forward).
Bottom line: there is no one 'quick fix' program. You need to tailor various programs to various travelers, at various stages.
Tough times for travel business, but those who are innovative and thinking ahead, can still do well.

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