Depends on how experienced you are with the mods... some of the advanced stuff can keep you occupied for days.
One thing I really love to build are rp2 frame quarries, I think they are really rewarding in terms of being useful as well as sense of achievement. As soon as I can lay my hands on some more copper, I'll rebuild an old cc-controlled design that I built in another world, but this time frube-optimized... Can't wait!