First, we rented a pipe snake from Home Depot to try to clean out the drains. This was not as successful as I'd hoped, unfortunately. It looks like we'll have to get a professional to work on those, which will be expensive. But this was a $36 bet that we'd be able to avoid paying a lot more. It was the right strategy for us to make that bet, even if it didn't pay off.
After concluding that that wouldn't work, we looked at other tasks. One task was to fix the garage door opener to work again. This turned out not to be too hard; just a matter of resetting the chain on its gears and adjusting the limits of the opener.
The other task was to replace the outdoor light in front of the garage. This light was in very bad shape. The support it was attached to had rusted out, so it was being held up only by wires. When we took a closer look, we discovered that those wires had been covered with cloth, and the cloth had long since rotted away.
So, back to Home Depot to get junction boxes and wire that could stand up to moisture. (I'd gotten the replacement light on the first trip.) This trip also included the purchase of a caulking gun and caulk.
We cleaned out the hole through the wall; I'll have to patch that hole in the concrete later. We put in a new junction box and filled the gaps around it with caulk. (The first time I'd ever used a caulking gun.) We wired things up and turned the power back on; the new light looks very nice. There's some follow-up work that needs to be done, but it'll do for now.
One side note: While we were at Home Depot, Les ran into a former co-worker of his. Jimmy had left employment at the hospital with Les to become an electrical contractor, then had been offered a job by Home Depot. He's not stocking shelves at Home Depot; he's paid to have a clue about hardware and the sorts of problems hardware store customers are facing, and to share that clue with people. I'm glad to know that Home Depot employs people like that; I think it gives them a much better chance of being able to serve my needs.