The huge number of cobbled streets, beautiful churches, half timbered houses with many colours, fantastic cafes and restaurants make Colmar one of the "must see" towns of Alsace. The old town has everything, including amazing canals of Little Venice. You can easily spend there several hours and you'll still be not bored.
You can easily park your car on the really huge parking lot called Place Scheurer Kestner (P1; GPS: 48.081927, 7.357209). There are of course another good parking lots you can use, for example the Parking de la Manufacture located north-west of the centre (P2; GPS: 48.083450, 7.353491), or undeground Parking Rapp located near Rapp Square, just next to the Old Town (P3; GPS: 48.076899, 7.352646).
After leaving the car you may walk the Rue du Rempart and Quai de la Sinn in the direction of the church of Domincans - Église des Dominicains (A; GPS: 48.078305, 7.357121) from 14th century, assuming you parked the car on the Place Scheurer Kestner or Parking de la Manufacture. If you parked the car on the Parking Rapp, please use the Rue de Boulangers or Rue des Tetes. Nearby the church there are restaurants where you can try Alsatian dishes.
The Cathedral Square - Place de la Cathedrale (B; GPS: 48.077569, 7.358168) is located just about 100 meters away from the Église des Dominicains. You can find there amazing St. Martin church - Église Saint-Martin, built in 13th-14th century. You can admire the coloured tile roof and walls in interesting colours.
Then you can go to the Lauch river (C; GPS: 48.074619, 7.360314) to visit so called "Little Venice" (La Petite Venise) via Rue de l'Eglise, Grand Rue and Rue des Tanneurs. Little Venice really resembles the real Venice because of interesting layout of houses on both sides of the river Lauch. Then you can try the boat trip (duration 30 minutes, price around 6 €) or just walk along the river. Beautiful colored houses are visible everywhere.