Snaefellsnes last day?

I would not do that as the testing center at Harpa does not open until at least 10 am, later on weekends, and you would want to get an early start to be able to do Snaefellsnes. Instead, I would suggest the Reykjanes Peninsula, which is the area south of Reykjavik. There is SO much to see in this area, but it isn't mentioned nearly as often as things along the ring road.

We just got back from our 3rd trip this week and we did a day trip to the Reykjanes and it was wonderful. You could test at 10am (when the site opens at Harpa) and then make the drive with plenty of time to be back in Reykjavik or the airport. We tested twice at Harpa and they are very quick and efficient. We had our results in 25 minutes both times.

Even in April, you may still have weather issues, so that is another thing to keep in mind.

Snaefellsness is a pretty long day trip, but doable, weather permitting. If you take a proctored home test with you to do in your hotel room and get an early start, you can do it especially if you see what time the sun sets and take advantage of evening light.
When I did this trip last August, I left Reykjavik about 8:30 in the morning. I wanted to skip the tunnel and drive around Hjallfjordur instead, because I heard that it was very scenic. I highly recommend taking that route at least one way. It took about 45 minutes to go around, and I stopped for pictures.
I wanted to hike between Hellnar & Arnarstapi (and back) on the end of the peninsula, so I stuck to the south side of the peninsula instead of driving the whole thing. I stopped at Budir, the black church, and then went on to Hellnar to hike. The hike took several hours, but is basically on a cliff over the ocean most of the way, and you should walk at least a little of it even if you want to keep traveling around the peninsula.
On the way back to Reykjavik I went around the fjord again, because it had been cloudy most of the day and the sun was finally coming out, making a beautiful sky over the fjord.
I was staying in Mosfellsbaer, on the outskirts of Reyk near the highway, and I got back about 8 pm. If you are staying near the airport that would add on another 45 mins to an hour to your trip back.
In a separate trip to Iceland, we also did a day trip from Reyjavik to Vik and back in a day, with a glacier hike. That trip was in June, so we had 24 hours of daylight. We had a great time, but that was a long day in the car. If you don't want to spend another long day in your car, you could check out the Reykjanes Peninsula which is 30-45 minutes outside of Reykjavik. The sights aren't spectacular like they are along the south coast, but there is a nice lighthouse with a view and I personally loved Kleifarvatn Lake and the Seltun Geothermal area. That's the area where volcano was erupting when I was there - not sure if that's still going on.

Mid April could very easily still have winter weather. I would not plan to drive all the way from Vik to Snaefellsnes in one day and then back to the airport. That would be a really long day in the summer, but not a good idea in April. When we did a bus tour of Snaefellsness from Reykjavik in the summer, it took 10 hours.

There really is a lot to do on the Reykjanes Peninsula. We spent a whole day there getting to KEF from Selfoss.

Thank you everyone