For ideas: write things down and regularly re-visit them, or they will grow stale and confusing. It will be hard to re-enter that topic usefully if you don't re-visit your old ideas.

For projects (e.g. programming) short-term interruptions are what people usually think about, but putting a project down for a few months will make it extremely hard to pick it up again, relative to putting it down for only a few days, even if working conditions are optimal. Continuity at all time-scales is important.