Hi Meg, I agree that one night is not enough on Lewis and Harris. But I spent two nights on Lewis back in 2023 and felt like I saw most of what I wanted. It did make me want to come back and I definitely will sometime because I love the island. But two nights gives you a good look at it.
If you're getting there in the evening, depending on where you are going, you might want to take a walk up to Lews Castle. I walked there from my B&B and it was a lovely walk through a huge forest, but very open and green. The castle itself has a wonderful museum. At the time I was there, they even had some of the Lewis Chessmen pieces, but I'm not sure if they're still exhibiting them. But even so, it's a wonderful place to visit and it would give you something to do that first evening. And given that it's July, you'll have plenty of light till 9:30-ish.
One advantage you have is that many of the main attractions on Lewis are fairly close to each other. So you can see a lot of them in a relatively short amount of time. While Frank is correct in that these are not highways, I did find the roads on Lewis to be fairly well maintained and easy to drive on compared to some of the other Highland roads.
As mentioned, I only had one full day on Lewis and pretty much drove around in a circle, seeing all the sites that were in the vicinity. I started with the Callanish stones, which were really interesting, then headed to Dun Carloway Broch (well worth a stop), and the Gearrannan Blackhouse Village (interesting but only reproductions).
The Shawbost Norse Mill and Kiln were really interesting, as well. To get there, you must walk up some steps made of flat stones, then walk about a half mile to the 2 buildings. I also really liked the Arnol Blackhouse and definitely is worth seeing. I drove to Port Ness and wanted to head out to the lighthouse, but due to construction, the road was closed so I didn’t make it all the way.