Our family thinks one day at DL is definately a day worth doing. Plenty of fun to be had. Checking out a map before and having a game plan might help. Enjoy your trip.
GO For it. Get a 1-day parkhopper. Since the parks are right across from each other you can visit both parks in one day easily. just pick a few "must do" things from each park. no need to make ADRs. if you only have one day i would not do any table service restaurants. Disneyland and California adventure have great counter service places, with lots of great options. just grab a quick snack and continue on.