I started doing intervals with heart rate, before buying a power meter. After 2 or 4 weeks off, I simply do endurance by HR, and some short tempo intervals with a HR cap. And toss in some 5-10 second sprints.
It takes a week or two for your blood to carry more oxygen and related endurance adaptations. During that time your FTP can increase quite a bit. The last time I took 4 weeks off, my estimate for ftp dropped to 200W (going by power-to-HR). Within 2 weeks it jumped up to something like 230W. This situation is the perfect reason to use HR (or RPE) instead of stressing about FTP changes.