Logging trees, making products out of those trees, then throwing those products in landfills. Works particularly well with paper products, as paper can be made from especially young and low quality trees.
Need more research but sounds like those are within one or two orders of magnitude.
Landfilling paper *is* geo-engineering, just particularly slow geo-engineering. Same as burning coal.
So may want to optimize short term methane production for fuel, while minimizing long-term.
Long term: as close to an inert, stable, pile of coal as possible.
It's almost certainly not possible to re-bootstrap civilization again the way it was bootstrapped the first time around, as all the easy coal/etc. reserves are gone. Landfills full of fuel could prove useful.