An inability to think ahead is a frequent problem for many people with inattentive ADHD and one that is directly related to the mental sluggishness associated with the disorder.
When your mind is feeling sluggish you tend to drift from one activity to another in an inefficient way. You also tend to miss opportunities to get more than one thing done at a time. However, if thinking ahead becomes a regular habit, than it should be easier for you to get past this mental sluggishness and get things done more efficiently.
A few years back I did a secondary teaching diploma. Unfortunately I haven't been able to find full time teaching work, but I do think the planning aspect of the course has helped some of my ADHD symptoms. Teacher training requires lots of lesson planning and thinking ahead, and since doing the course I find I'm less likely to forget things I need, and generally find it easier to get started on tasks requiring forethought.
I guess any course or activity which involves lots of planning or thinking ahead would be beneficial. Courses in building project management, restaurant or catering management or just organising recreational trips for family or friends are some of the other options to consider. Games like chess, where you have to consider all the different consequences of your actions, might also be useful for stimulating the parts of your brain involved in planning and forethought.