We went there early October. While tree foliages have just started showing autumn colors in some areas in Seoul, the trees in Nami Island were already showing more oranges and reds, and that really completed our trip since we travelled during early part of autumn. :D You can get to the island via a quick ferry ride or via zipline over the waters (more expensive but a good experience if you have not done ziplining) I cannot remember seeing resto/cafe in the island itself, but there are places where you can eat near the docks going to Nami Island. Remember to bring your passports (for non-Koreans) for ferry ticket discount.