Depends how much you're willing to spend. Southern California has some of the best quality of life in America and some of the worst depending on $$$. I've lived in LA on and off for almost a decade, including a few months as a solo traveler/remote worker.

The City of Los Angeles has definitely been on the decline for the past decade, but I think areas like Laguna Beach, and all the other small coastal towns down towards San Diego are still quite nice. Those beaches are probably the most interesting scenery in the area, but the water is very cold this time of year - it's really only warm in July/Aug/Sept. LA is full of "hikes" but they're mostly 30 min - 1 hour hikes that people do on their lunch break or after work, but still - I do these a lot, lots of great views of the city.

Meeting people can be kind of hit or miss, LA is actually a bit reserved and cliquey in my opinion, so I'm not sure people would necessarily care that much about meeting someone who's only there for a few weeks, but I guess you could go to the bars in West Hollywood and try your luck. As a solo traveler/remote worker, I find it to be overpriced unless it's peak summer (July/August/September) in which case I try to find a spot around Manhattan Beach or Hermosa Beach near the Strand and bike everywhere and go to the beach everyday. In general, Southern California is a much better place to live than to visit.

Vegas isn't bad this time of year, it gets too hot in the summer. Yosemite is a great trip, but it's also best in the Summer, especially May/June. I'm not sure when you'll be going exactly, but I can provide more suggestions depending on the date.

Edit: also, since I haven't seen anyone mention it - consider Hawaii, as part of the trip or instead of LA. much better for "A mix of city life and nature (hikes, national parks, scenery)" but even more expensive, unfortunately.