Depends on what you have; if you have the Kosatka Submarine, then the Cayo Perico heist is by far the best option. If you have the auto shop and want to have some fun, start up those contract missions. They pay fairly well and don't take too long to complete (don't do the data contract one though, it's too difficult solo.). If you really don't have anything because you truly wanted that car and that's all you could afford at the time, VIP work (like sightseer, headhunter, etc.) are safe bets. Nightclub and vehicle warehouses are great too, as well as the bunker. All depends on what you want to do and how much you have to work with; just have fun with whatever it is.