Add sprints that have multiple segments and rest areas between the segments. This will be a fun and competitive way to do intervals. You will be timed for the period going through all the segments, but not between that segments. You should be able to pedal easy between segments, but there should be a maximum time you can be in a rest area, perhaps at speed requiring about 1/3 to 1/2 the watts averaged in the previous segment. As with other sprints and climbs, there should be a leader-board, and you should be able to compare your current effort to past efforts. There are unlimited variations that can be built , but one example would be a set 6 of 300 meter sprints with 400 meters of rest between the sprints, using a total of 4200 meters of road.