While not strictly a 'day trip' since Belmont Shore is actually just a nickname for a district in Long Beach, if you're staying in downtown Long Beach make sure to take a drive/bus to Belmont Shore for a day of shopping. There are unique bistros, trendy clothing stores, and home decor stores like Romance etc. There are many outdoor eateries like Open Sesame which makes this area ideal for people watching.

After shopping, drive one mile east on Second Street over the water to the Naples canals. You can stroll down the sidewalks right in front of some of the most beautiful homes in Long Beach, but these have back doors facing canals, most with boats moored there; Zodiacs, rowboats and even commercial gondolas. You can arrange for a private gondola ride if you have time.  Newcomers to this area feel as though they are walking a straight path and are often surprised to hear the canals form a complete circle.  The entire walk takes 40 minutes.

You can continue on Second Street in the same direction and will end up on Pacific Coast Highway and shops, restaurants, and theaters to your left and right.  If you're looking for groceries there is a Trader Joe's and a Whole Food Market.

If you want more beach and less shops turn back toward Belmont Shore and make the first possible left turn and will drive through a neighborhood where the houses are tiny and expensive because when the street ends you will be at Ocean Blvd. and one of the nices stretch of beach in Long Beach.  Small waves due to the breakwater, this is a sunbathers/splashers paradise.

In summer off Bay Shore Avenue on both sides of Second Street is closed to vehicular traffic because the beaches along it are so full of pedestrians it's impossible to drive it.  Check out the house on Bay Shore that face the beach and imagine what summer must be like living here.