Being a native so.cal girl, I love the beach any time of the year. But, people that visit sometimes have the perception that it's always warm. In early March, it's likely to be cool, particularly down by the water. Cool is a relative term, but I'd say mid-60's to high-60's will be pretty common, and much higher than that is possible, but not terribly likely. I just went on weather.com just to make sure my recollection wasn't all messed up and it shows 69 as the average high for both Feb. and March, with 49 & 47 being the average lows. Just to put things in perspective. The ocean breezes can make it even cooler. IF that is just fine for what you were wanting to do at the beach, then go for it, just wanted to make sure you knew what to expect. We often had family from MN and IA come to visit us during spring break expecting it to be bikinni weather, and it wasn't (we'd always tell them that's what Palm Springs and Cabo are all about!
) The ocean will definitely be too cold to go into unless you have a full wet suit.
If I were going back for a trip with my DH without the kids, I'd stay at the Roosevelt Hotel in Hollywood. It's steeped in history, has a really cool pictoral of the history of Hollywood. Plus, that's the area I'd want to be in. In the heart of Hollywood and all the Hollywood stuff to see and visit, close to Melrose, close to Beverly Hills. And then I'd go visit the beach for part of a day/evening. Like an afternoon/evening there, but most of my focus up in Hollywood area. It's only a couple of stops away from Universal on the metro. But, some people really don't like Hollywood.
I'd also love to stay on the Queen Mary in Long Beach, but there's not really anything around there to recommend.
Have fun! I think adult time away is very important for a relationship. Gives you time to recharge your batteries and reconnect with your spouse! We typically go away for a long weekend every year for Presidents day weekend, and my parents have always watched the kids. We miss the kids, but not enough not to go.
If you have any other questions, please let me know. There are alot of ideas on this board. Check out some of the other posts about what to do, what to see. Lots of locals responding. Some of my favorite restaurants talked about.