It may be a bit too far for your needs, but we just stayed at Asilomar in Monterey and it was really lovely.
Asilomar Conference Grounds in Pacific Grove (just past Monterey). It is a campus of lodging buildings, conference rooms, a social hall, and a dining hall set just inland from Asilomar State Beach. Sand dunes with a boardwalk separate the grounds from the beach; it's maybe 1/4 mile to the beach. The dining hall serves 3 meals a day. The social hall has a big fireplace, pool tables, and board games. The grounds are gorgeous and you will likely see deer wandering around sometime during your stay. Since it is set up for conferences of hundreds of people, handling 50 will be easy-peasy for them. The only issue is that you may have to schedule around any larger conferences. Many of the lodging buildings have common rooms, often with a fireplace. There are a variety of reservable spaces if you need something more private than the social hall or common areas. Oh, there's an outdoor firepit, too. They have some handicap accessible rooms (I took my mom there when she was 89 and using a wheelchair).
And of course Monterey is beautiful and has a ton of things to do, including the aquarium.
We had a 50+ person family reunion (ages 1-90) this summer at Asilomar (near Monterey) and it was pretty good. I don't know the price because my parents covered it, but they're usually pretty price-conscious. It has nice old buildings (many designed by Julia Morgan), so it feels more like a resort than a conference center. Everyone was housed in buildings near each other and we ate together in the dining hall (food just okay, not spectacular, but no cooking or cleanup - yay!). We rented a meeting room for one day, but other days we met up in the fire pit area or the lobby of the main building (which has a great woodsy lodge-like atmosphere) or other community spaces. The beach is right there, the aquarium is nearby, lots of room for kids to run around, board games to borrow, etc.; it felt like a real vacation. They had an on-call shuttle to help older family members get around the property, which was super helpful. We also considered Leonard Lake Reserve (Leonardlakereserve.com), if Ukiah's not too far away. They have 4 houses you can rent as a group and came recommended by Sunset Magazine.
I did a family vacation with my 89-year-old, wheelchair-bound mother to Asilomar Conference Grounds in Pacific Grove. They have accessible rooms (you'll have to call, as the online reservation system wasn't specific enough) and an accessible pool, and they lend beach wheelchairs to take to Asilomar State Beach (a short walk away). I have great pictures of my mom on the beach for the first time in 20 years. The grounds are gorgeous, there's a social hall with pool, puzzles and games, and a big fireplace, a dining hall that serves 3 meals a day (for an added cost), and a boardwalk on the adjacent sand dunes. The only downside I see is that you already did Monterey, and this is right nextdoor, so you may feel you have been-there-done-that.
If you're up for another trip to the Monterey area, we had a family reunion at Asilomar a few years ago. It worked well for kids and an adult with mobility limitations. Asilomar even has a beach wheelchair for guests to borrow. Come to think of it, many of the beaches in the State Parks system have beach wheelchairs.
For day trips... a lot of museums and other venues have loaner wheelchairs for guests who have trouble walking/standing for extended periods of time. Check the accessibility info on their websites. Maybe a ferry ride + pedicab + loaner wheelchair for an outing to the Exploratorium? Depending on your budget and your dad's comfort level, you might also get a walker with built-in seating (search term "rollator" on Amazon or Craigslist). After much coaxing, we're finally getting my dad to use one.
Archived Q&A and Reviews
Re: where to stay in Carmel
You might want to look at Asilomar Conference Center. It's not right in Carmel, but is right next to it, a very short drive. Great prices, full cooked breakfast, no 2 night minimum stays, gorgeous grounds. It was designed by Julia Morgan and is a really unique place. Rooms are simple and rustic, no TV's. Great place for kids too, with ranger-led campfires at night, a pool, an interesting nature center and tidepools right across the street. You can get more info & check availability at their website, www.visitasilomar.com. We've really enjoyed our stays there over the years.
Re: Monterey Lodging
See if you can get a room at the Asilomar conference center in Pacific Grove. They mainly book groups but will rent out individual rooms if they are available. The rates are very reasonable (and I believe include full breakfast in the cafeteria), the location is stunning and buildings lovely, and there's lot of running around space and paths to the beach. It's the region's great hidden deal! budget traveller
Re: Weekend of solitude for mom (March 2003)
You should consider Asilomar Conference Center in Pacific Grove (Monterey Bay). It is right on the beach, has very reasonably priced accomodations (though not fancy), and is extremely peaceful. It is near the Inn at Spanish Bay, which probably offers spa services. Their website is www.asilomarcenter.com. Meredith