Where to meet up in the Loop

The lit CHICAGO sign and marquee of the Chicago Theatre on State Street at dusk, with the elevated 'L' tracks and a Marina City tower behind — The Chicago Theatre marquee on State Street, seen from Benton Place. Photo by The Other 95% (CC BY 4.0) via Wikimedia Commons

The Loop is Chicago's main business district, inside the ring of elevated 'L' tracks. It suits plans that start at the office, like a team lunch, a client meeting or drinks after work. The theaters on State Street and Randolph Street make it a good choice for dinner before a show, and the Art Institute and Millennium Park suit a weekend afternoon.

Landmarks that are easy to name in a plan:

the Chicago Theatre marquee on State Street, just south of Lake Street
Cloud Gate, the mirrored sculpture in Millennium Park off Michigan Avenue
the Picasso sculpture in Daley Plaza, on Washington Street at Dearborn Street
the lion statues in front of the Art Institute on Michigan Avenue

Getting around

Clark/Lake, at 124 W. Lake St., has the Blue Line in the subway and five elevated lines above it. Its station page lists three entrances on Lake Street that stay open 24 hours.
The Red Line runs under State Street, with stops at Lake, Monroe and Jackson. The Blue Line runs under Dearborn Street, with stops at Washington, Monroe and Jackson.
On Wabash Avenue, the Brown, Green, Orange, Pink and Purple lines stop at Washington/Wabash and Adams/Wabash.
State/Lake is closed for reconstruction into 2029. Its station page points riders to Clark/Lake or Washington/Wabash, and says to change between the Red Line and the elevated lines at Washington/Wabash.
Metra stops at two stations inside the Loop, as its rider FAQ says. Metra Electric trains use Millennium Station at Randolph Street and Michigan Avenue, and Rock Island trains use LaSalle Street Station at LaSalle Street and Van Buren Street.

At night and on weekends:

The Red and Blue lines run at all hours, every day, as the Red Line page says. For a late plan, pick a spot near a Red or Blue stop.
The elevated lines stop overnight. The Brown Line timetable has its last trains leaving the Loop around 2 a.m. on weekdays and Saturdays, and earlier on Sundays.
The Purple Line runs to the Loop only during weekday rush periods, as the Purple Line page says.