Really, get a friend to do Dung with you. If you guys do a 2 man C6 small on all of your floors for something ilke a few hours you could get to level 45 probably. It's one of the fastest skills to train at lower levels, gaining multiple levels per floor. I got my friend from level 19-30 doing one large floor.
Construction: If you have 6-7m lying around you could get that to 70 within 1-2 days time easily. If you go a little cheaper, i wouldn't suggest 70 as it will get a little slower at the end.
Making Energy Potions for herblore isn't costly either, and can rack in some great exp. Try doing those from level 30 to like 50 or 60.
If you have money lying around, frost bones on a gilded altar would pretty much make you the most levels the fastest. you'd be gaining from 1-2 levels per load of bones where you are now.