You should also visit a Los Santos airport. You can find there planes like Luxor. What is more, you can steal big planes similar to Boeings 747 which cannot be bought in any other way. Unfortunately, you can't store them because they are too big for any hangar. There is also no entering them animation. To get inside of a one, you have to stop at the wheel and press the button for getting inside the plane.
Remember also, that if you don't have a hangar, getting on the airport gives you three Wanted stars.
Maze Bank Tower
The highest building in LS, inspired by US Bank Tower in Los Angeles. Wonderful viewpoint, what is more, there is a Helipad on the roof (you can't buy it). It's located in the Pillbox Hill, in the center of the city. If you can't find it, just look up.
On the northeast end of LS you can find a huge VINEWOOD sign, inspired by famous HOLLYWOOD. You can climb on one of letters and see a beautiful panorama of the whole city. What is more, from the top you can easily notice nearby race track, amphitheatre and observatory. Those are next interesting places which you can visit and where you won't get during the main missions.
On the beach in Del Perro you can find a fun fair. You can use some attractions of it, like the Ferris Wheel to look on the city from another perspective.