A very nice trip you have planned...
I think that if you're not a hiker or an outdoors person, or a photographer, one day in Yosemite might be enough (that is, from early morning till dark). It's gorgeous there, but if you are only going on the trails near parking, you'll be able to see things in one day. If you like scenery and history, Highway 49 is the way to go from Yosemite area to Placerville. In mid-May, the hills will be green and lush. Expect fluctuating temperatures. Lots of historic gold rush towns along the way, from Jamestown to Jackson to Sutter Creek and Placerville. All of these towns have neat old main street downtowns that are lots of fun. You'll find gold panning and gold mine tours, indian casinos, shopping, and bed and breakfasts. Coloma, where James Marshall discovered gold, is nearby and is a state park. In Placerville, the downtown is very vibrant. Try this site:
or this one:
In Placerville, I recommend Sweetie Pie's for breakfast, in nearby El Dorado (Hwy 49 + Pleasant Valley Rd) I recommend the famous Poor Red's BBQ, and in nearby Diamond Springs I recommend Diamond Chinese (absolutely no ambiance, but popular with locals - the cherry chicken is yummy).
For Jackson and Sutter Creek, you might want to look up Amador County websites:
Both counties have beautiful scenery and lots of wineries and farm trails. As for amount of time to spend, it depends on how much you like to mosey along!! I think at minimum 2 to 3 days.
Head up Highway 50 eastbound towards South Lake Tahoe. East of Placerville is Camino, home of "Apple Hill". Lots of farm trails here, some farms may be open...Boa Vista is always open. Check that "visit el dorado" link. Pollock Pines is at the pine line. Take a deep breath, and enjoy the view of the ridges. Do pull over for Bridal Veil Falls for a photo, it's very pretty and no hiking involved. All this will take you only a few hours (from Placerville to South Lake Tahoe). Lake Tahoe...it will take a day to drive around. Too cold for swimming in mid-May. Lots of places to explore by car,though. Gorgeous! Places to eat...for informal lunch or dinner, try Izzy's Burgers on the South shore on your way to the Nevada side of the lake. Great burgers, fries, and malts. I usually recommend Womack's BBQ,it is southern style, but that might not be what you want on vacation (since y'all are southerners)...if you are going to get a buffet at one of the casinos, DON'T do the Horizon buffet, yuck. If you are willing to shell out $26/ea for dinner (less for lunch), the Forest Buffet on the 18th floor of Harrah's is outstanding in food and view...especially at sunset. On the northwest side of the lake, in Tahoe City, I recommend the Sunnyside Restaurant (and it looked like a lovely hotel, too). So, depending on what you want to do in Tahoe, at least 2 days.
Back down to Sacramento...you could actually go down Highway 80 from the north side of the lake, if you don't want to repeat the same stretch of Highway 50. In Sacramento there is a lot to do. I recommend the free Capitol tour. The capitol is a beautiful building, just restored. The gardens are outstanding and free as well. Also in the city is the incredible 1800s era just restored Cathedral of the Blessed Sacrament, also downtown, as well as the just-opened newly restored Stanford Home. There is also the Victorian Governor's Mansion, Sutter's Fort, the Indian museum, and the Railroad Museum (which I found fascinating, and I'm not a railroad buff). There is Old Sacramento, the Crocker Museum, the Towe Auto Museum. Try this link:
There are some neat hotels, like the Sterling Hotel near downtown. A Sacramento tradition is Frank Fat's, whose chinese restaurant has been open since the mid 1800s. I would think a minimum of 2-3 days in Sacramento, again, depending on what you want to do. A secret that nearby state workers know about...shhh...on 10th Street (one-way, north) between R Street and P Street, on the right hand side of the street, is a fortune cookie factory. Walk inside the screen door, and you'll be greeted, and ask for a bag (or more!!) of their "TV cookies", which are their irregular cookies, but very tasty, and a bargain at $1 per bag.
Well, for me, I would do 10 days to do this trip. And no hard and fast itinerary, just do it as you find it, and enjoy it. You'll not be in "high season" yet, so there shouldn't be much competition for hotel/motel rooms. Waterfalls in Yosemite should be great at that time of year. Have a great time!!