I've done some tactical urbanism in the past. If you want to do it with the city's blessing, here is what I would say:
1. Come up with a semi-firm proposal first. Draw it out so people can envision it. Don't make them design it. Make it easy for them. Most city employees I've worked with will NEVER do anything that adds more work to their plate. Make it as CHEAP as possible and pay for it all yourself or with your friends if you can.
2. Don't make it political. Bring it to whoever is in charge of streets. Maybe planning or engineering; find that out. If they don't respond then maybe you can take it to a city council member but NOT at a public meeting. Just private one-on-one. Putting them in public in front of people with their peers is a quick way to make it political and get the idea shut down.
3. Make it clear that the city can back out at any time if something goes wrong. City staff are almost always scared of problems. Knowing that they can quickly and easily reverse the project brings them peace of mind.
And lastly, there's always the option to go without the city's blessing if you don't think they'll go with your idea. That is perfectly fine but don't do anything that would put you in jail. Be careful.
Also if you need help with your design idea, check out this site: http://tacticalurbanismguide.com/ 15
also this one: http://urbanrepairs.blogspot.com/12