We were on Monarch last January, hubby, me and 2 small kids in an inside quad room. We did the 4 day.
We loved Monarch, thought she looked really well, we were happy with the service. Thought the food was actually better in the buffets than in the dining room. My husband liked the sushi in Jade (there's a charge for it, but he thought it was worth it). Kids had a blast in Ocean Adventure which wasn't crowded because it was not a school holiday.
Our cabin carpet was wet, when we told our cabin steward, he took care of getting it dried out. Then we discovered why it was wet! The bathroom drains were clogged and if not careful, the water spilled out of the bathroom . I don't know why the previous occupants didn't report this, but we did, and maintenance took care of it. We placed the call to maintenance, but then also told our cabin steward who was great about following up on it to make sure it all got fixed, it did that day.
The inside cabins are quite small, but organizing all our stuff the first day was key, using the small drawers and any sheving in the closet helped, put the suitcases under the bed and dirty clothes went directly in there, then as the week progressed all was well.
As far as activities, have rented golf carts and also booked tours on the pier (not through the cruise ship) on Catalina. Also, I always stock up on bottled water and snacks and anything we may have forgotten at the Vons on Catalina to take back to the ship. The bus tour takes you up the hill and you get a really lovely view of the island/harbor from up there plus learn a bit about life on Catalina and it's history. RCI has really nice ramps to the tender boats (unlike Carnival which you have to step on to a bobbing boat -- a real challenge for my elderly mother, wish I'd have known about that difference)
Ensenada, one of my favorite things to do is take the La Buffadora tour. Again, I just book this from the tour operators, not cruise ship, safe reliable buses, great fun shopping at all the stalls on the short walk to the Blow Hole (Buffadora) (jewlry, leather goods, linens, etc, barter for great prices) and beautiful scenerey, the guides tell you about Ensenada and the area, v. interesting.
San Diego, well been there lots of times, so just strolled a bit and then took a taxi to the shopping plaza.
You'll have a great cruise, the Monarch is very nice looking ship.