Sprints definitely. I would sprint between two spots and go longer each pass. For example on an American football field, run from the 50 yard line to the 40 yard line, stop, run back to the 50, stop run to the 30, stop, run back to the 50, etc. As for being embarrassed, you'll have to overcome the feeling if you want to be a champion. I remember watching a documentary about Arnold Schwartzenegger. He would watch his competitors do their lifts. The ones who would puke and stop were the ones he knew he was going to beat. You have to be able to push through the pain, whether it be physical or emotional. Do what it takes to succeed. I think you'll find most people at the park won't even think twice about it, especially if you are a regular.
Short intensive bursts. Practice sprinting with a ball under your arm...it's easy to run fast without it; not as easy when you're protecting your 'baby'. I always found it helpful to sprint against someone or something (a dog is VERY useful or even a passing car) It will be embarrassing for the first time or two but when your mind gets into mode, you'll forget about them fairly quickly. Plus, girls like watching men train and exert themselves.
Hill sprints or stair sprints work well.
Up the stairs? Through alley way? At night? Any of those help?
