If you do want to park downtown, use a parking app like ParkWhiz or SpotHero – it will save you a lot of money and frustration finding the most affordable places to park. I drive downtown all the time and would never park anywhere without first booking a spot using the app. You can find spots for $10-$15 for most of the day often with it, where you’d normally pay $25-$50+. (For example, right now I can find a few spots within a block of the Art Instititute for $12-$13 for 12 hours of parking.)
That said, the Orange Line is convenient – I live one stop from Midway – about 20 minutes-ish to get to downtown and that train line circles the Loop, so another 8-12 minutes or so to get to the northeast end of the Loop tracks (where you’ll want to get off if you want to hit Millennium Park.) For touristly stuff, I might get off at State & Lake, take some pics in front of the Chicago Theatre, walk up north to the river, go down the riverwalk and walk east to Michigan Avenue, take in the nice view of the Wrigley Building, the former Tribune tower, and even the Trump tower is quite nice (minus the giant Trump letters) – walk up to Michigan Ave., then walk south on the east side of the street to get to Millennium Park. If you walk north instead, you get to the “Magnifiscent Mile” stretch of Michigan Avenue, which is mostly a shopping district. I don’t find it personally that interesting except for the bustle, but if you walk far enough north, you’ll get to the Water Tower. Unless you want to do a good bit of extra walking, I’d just go south, hit Millennium Park, and I personally love the Art Institute. Further south yet is the museum campus, which has the Shedd Aquarium, the Field Museum, and the Adler Planetarium. The strip of land between the Shedd and Adler provides a nice panorama of Chicago, and one of the “postcard shots” you see of the city. That whole walk would be about 2 1/2 - 3 miles (without detouring north on Michigan.)
There are other cool spots within the Loop to see – the Chicago Board of Trade, maybe the Picasso at Daley Center depending on what you feel about it – there’s a nice lChagall mosaic titled “Four Seasons” at the Chase Tower Plaza that I personally quite like (20 S. Dearborn). Oh, the Chicago Cultural Center is free, and that’s on Michigan and Randolph.