Most people know I'm a big fan of bike trainers. They are a very safe and time efficient way to include focused interval work into your schedule. No worries about cars, weather, road conditions... just hop on and hit the power targets. A good bike trainer can be used to build or maintain power year round.

That said, it's not unusual for trainer motivation to wane when it's beautiful outside. Just because you are taking a break from the trainer, it doesn't mean that you can't continue to build quality work into your schedule with focused intervals. With a little thoughtful planning, you can do intervals and enjoy the outdoors.

Here's some tips on doing interval work outdoors:

Ride safe. ALWAYS ride aware, and slow down and/or adjust your power/speed based on the situation (traffic, road conditions, etc.). Making your intervals look perfect is not worth an accident, period.
Choose an appropriate route... ideally with very low or no traffic.
- Bike trails. Some bike trails can work well for intervals. In particular, flat and straight routes without a lot of other cyclists can be ideal (e.g. some sections of the High Trestle Trail). This can give you a long sight line, to ensure you can safely maintain higher power / speed. Obviously, trails with a lot of curves, limited sight distances, frequent intersections and a lot of walkers would not be a good choice.
- Hilly county roads. Hill repeats on a quiet road with a decent shoulder can be a good option as well. This can provide you with a consistent way to hit your target power on the climbs, and use the descents as your easy recovery.
- Gravel. Yep, these days I do most of my outdoor intervals on gravel. Generally gravel is much quieter with significantly less traffic. It also creates additional resistance, so speeds aren't as high and it's easier to maintain steady power. Obviously you need to make sure you have suitable conditions where you can maintain good control of your bike.
Faster isn't better. The point of intervals isn't speed, but rather focused power (or HR) targets. I've found it easier (and safer) to ride "slower" bikes for interval work, to help maintain steady power and avoid excess speed (particularly on trails). Examples are using a gravel bike with aerobars rather than a TT bike, or using a fat bike instead of a mountain bike, to find slower (meaning greater aerodynamic and rolling resistance) options with at least similar geometries.
Keep it simple. Anyone who has done some of my interval workouts knows I can get creative at times with multiple target intensities within a single high power block (e.g. mixing VO2 and sweet spot work, etc.). While these can provide some incremental benefits, they are easier to do on a trainer. Outdoors, it is generally easier to execute (and remember) intervals composed of simple blocks (e.g. 6 x 7 minutes at 90% power).
Technology can be your friend. In the past, I would print out my workout and tape it to my bike to stay on track. Many of the more recent bike computers have workout modes. Personally, I have both a Garmin 820 and a Wahoo Elemnt Bolt, and really like the Wahoo for doing structured workouts. Although you can sync workouts from TrainingPeaks on both devices, the Wahoo's default workout screen gives you a nice visual of key metrics from the workout (see below). I find this works very well for me to stay on track.
Accept imperfections. This tends to be the hardest item for most people I coach, as most are type "A" people and want their workout data to look perfect. ;) In the end, it doesn't have to be spot on with the plan, to still get a valuable workout in. It's fine to have some variance with some blocks a bit higher than others, lower recoveries, longer / shorter blocks when you need to accommodate your route, etc. It's way more productive to have some structure than no structure, while still enjoying the outdoors.