What short-term alternatives do airlines have to replace their grounded Boeing MAX 8s?
Airlines really don't have a lot of options to carry through with the flights they planned with the grounded airplanes.

Older models of the 737 aren't much of an option. Most of the MAXes weren't intended to replace retiring aircraft, and even if they were, various costs of bringing the older aircraft into service for just a few months would be too expensive to make sense in the long run. The A320 would be the most direct replacement, but even then the situation is similar. New aircraft are ordered years in advance, so they can't just go buy an A320neo and get it right now.

It's possible to use a larger aircraft like the 757, but they would have to be pulled off another route and would burn more fuel.

The summer is peak travel season, so there just isn't any spare capacity out there that's economical to bring on board on short notice for just a few months. Even if they did, once the hundreds of MAXes return to service, the excess capacity is a liability.

Aircraft are a very expensive asset. Fleet needs are planned well in advance to have only as many as they absolutely need. Even an unexpected increase in maintenance time causes cancellations. The downside to this is that they're in trouble if a large disruption hits. The cheapest option is to just cancel a lot of the flights.