Lake Springfield gives Illinois' capital a broad outdoor playground south of town, with marinas, parks, fishing areas, and long water views. It was built as a city utility reservoir, but for visitors it works as a low-key recreation area.
Come in warm weather if you want the lake to feel active. It pairs well with Lincoln-heavy sightseeing when you need a break from museums and historic houses, though you will need a car to make good use of it.