I would suggest playing some very diatonic stuff, children songs, pentatonic tunes, etc., because the consonant intervals help building up a solid intonation. To play in tune, you must be able to pre-hear and sing what you are about to play. You can use a drone or a tanpura while playing scales, arpeggios, etc.
Use for reference open strings and harmonics as they are automatically in tune. You can play lines in which fingered notes precede or follow the same note on open string or harmonics.
First practice with no shifts. Practice playing fifths, octaves, major and minor thirds, major and minor seconds in fixed position to help establishing a solid hand shape. Simandl technique (1-2-4) is good because the relaxed hand falls quite naturally on a whole ton between 1 (index) and 4 (pinky), with 2 (middle finger) playing the half tone in between.
To work on shifting and pivoting, practice finger-replacement shifts, where for example your index finger replaces your pinky playing the same note. The unison is the easiest interval to play in tune.
Try doing this with closed eyes very slowly focussing on what you ear tells you. Sure, in actual playing, you will use the lines and/or dots to guide your hand, but the less you need them, the more you will be able to play in tune.
Playing fretless is a wonderful experience of ear training. Enjoy it!
