Lavender is a Mediterranean plant -- well adapted to hot, dry summers. Hydrangea isn't. Unfortunately, plants in pots that are allowed to dry out don't get just a hot dry summer experience, they get a middle of the Sahara experience. Keep watering, and they may come back, though.
Can you group the plants together in a plastic tub or small wading pool of some sort? That makes it really easy to water... just dump a few milkjugs full of water over the plants, and let the extra water (which should just be no more than a half inch deep) take care of the next day or so's watering,
Or you could set up a wick watering system for the plants.
Another choice, particularly appropriate if they're in clay pots, is "double potting": putting the clay pot in a larger plastic or ceramic pot and stuffing the space in between with long strand sphagnum moss (not sphagnum peat nor peat moss). The sphagnum soaks up the extra water that runs out of the pot and then allows the clay pot to take that water back through the walls of the pot as needed.
Search "double potting" here:
http://pubs.caes.uga.edu/caespubs/pubs/PDF/B737.pdf
wick watering
http://www.ajc.com/living/content/living/homeandgarden/stories/reeves/2008/10/30/walter_waterer_wick_hydrated.html
Or, if you're out of energy, perhaps it would make sense to pass these plants along to someone who has the time? Perhaps a family member or neighbor? Gardening can wait for a year or three.
