It might help if the OP was still around. I suspect that he/she has left us as a result of nobody being willing to read the Owner’s Manual for him/her.
For anyone still interested, here’s a link to the Smart USA website with the checklist for the 20,000 mile check, and associated links for other maintenance schedules.
The problem is though, while they say you can DIY and tell you what needs to be done, they don’t tell you how and don’t make the information available to the public including independent mechanics. I’m told by Penske Auto group that you can’t download or print from their service manual online that costs a hefty subscription fee.
Take it light.
Mark