Probably commodities, especially non-renewable resources, and companies that produce them.

If we were to go into a long slump with slow technological growth and minimal infrastructure development, then that would probably mean we would continue to use fossil fuels for a long time, which would have to mean steadily increasing prices over the long haul as supplies slowly get smaller and more expensive to produce.

The downside is that if the long slump doesn't happen, if technology does continue to advance and infrastructure investments in better technology are made (electric cars, high-speed rail, renewable, third generation nuclear plants, maybe even fusion if we're lucky), then the value of those may suddenly drop at some point. But I think your "long slump" scenario is assuming that doesn't happen; significant technology change and re-building our whole infrastructure would almost certainly mean economic growth.

I'm not sure if it pays off 100 to 1, but it should give a good rate of return over time in a long slump scenario.

I don't think the long slump scenario is very likely, but that's a different issue.