I don't know what you mean by beach time? If you mean swimming in the ocean, then you won't find any beach time in San Francisco during March or April. You might find some reasonably comfortable beach lounging weather in Los Angeles or, even better in San Diego, but swimming will still be rather invigorating. The surfers will certainly be in the ocean, but they will be wearing wet suits.
At that time of the year, nearly all the passes across the Sierra Nevada Mountains will be closed due to snow accummulation. You can drive southwest from Las Vegas to Barstow or to Los Angeles, then follow the Central Valley or the Coastal Highway north to San Francisco. Otherwise, you must drive northwest from Las Vegas to Lake Tahoe or Reno and take route 80 through the Donner Pass that is kept open all winter. Either way, it will be about a two day drive with an overnight stopover along the way.
The northern route offers a lot of desert scenery and some snow covered mountains, but the southern route along the coast provides some great coastal scenery like Big Sur, Santa Barbara, Monterrey. If you go up the Central Valley, you can go to Yosemite, an exquisite mountain valley, but that will require you to venture into the snowy mountains.
If you want to chill out and take it easy, I would suggest you take one of the tours from Las Vegas to the Grand Canyon, or even drive five hours from Las Vegas, over boulder Dam, along the remnants of route 66 and to the South Rim visitors center. You can return to Las Vegas and catch an airline shuttle to San Francisco, then spend a day or two exploring that very picturesque city.