using the 2 techniques you both suggested, and Yamllint checked, seems to be fine both ways. The initial state is there as an example, and ‘pattern’ checks if one fills it put correctly 
full package fyi:
##########################################################################################
# Package Google Traveltime
# only feed api with calls while necessary/desired to mitigate api call restrictions
# MtHvdB 07072018
##########################################################################################
homeassistant:
customize:
sensor.travel_destination:
templates:
icon: >
if (entities['input_select.travel_mode'].state === 'bicycling') return 'mdi:bike';
if (entities['input_select.travel_mode'].state === 'driving') return 'mdi:car';
if (entities['input_select.travel_mode'].state === 'walking') return 'mdi:transit-transfer';
if (entities['input_select.travel_mode'].state === 'transit') return 'mdi:train';
return 'mdi:bus';
sensor.select_travel_destination:
icon: mdi:login
sensor.select_travel_origin:
icon: mdi:logout
sensor.select_travel_mode:
icon: mdi:transit-transfer
input_select.travel_mode:
templates:
icon: >
if (state === 'bicycling') return 'mdi:bike';
if (state === 'driving') return 'mdi:car';
if (state === 'walking') return 'mdi:transit-transfer';
if (state === 'transit') return 'mdi:train';
return 'mdi:bus';
sensor.distance_selected_destination:
icon: mdi:map-marker-distance
friendly_name: Distance
sensor.duration_selected_destination:
icon: mdi:timer
friendly_name: Duration
sensor.duration_in_traffic_selected_destination:
icon: mdi:traffic-light
friendly_name: In traffic
##########################################################################################
# Sensors
##########################################################################################
sensor:
- platform: google_travel_time
api_key: !secret google_travel
mode: sensor.select_travel_mode
departure_time: now
name: Travel destination
origin: sensor.select_travel_origin
destination: sensor.select_travel_destination
- platform: template
sensors:
select_travel_destination:
value_template: '{{states.input_select.travel_destination.state}}'
select_travel_origin:
value_template: '{{states.input_select.travel_origin.state}}'
select_travel_mode:
value_template: '{{states.input_select.travel_mode.state}}'
distance_selected_destination:
value_template: '{{states.sensor.travel_destination.attributes.distance}}'
duration_selected_destination:
value_template: '{{states.sensor.travel_destination.attributes.duration}}'
duration_in_traffic_selected_destination:
value_template: '{{states.sensor.travel_destination.attributes.duration_in_traffic}}'
input_travel_origin:
value_template: '{{states.input_text.travel_origin.state}}'
input_travel_destination:
value_template: '{{states.input_text.travel_destination.state}}'
##########################################################################################
# Inputs
##########################################################################################
input_text:
travel_destination:
icon: mdi:login
name: Travel destination
pattern:
- City
- Country
initial: "City, Country"
travel_origin:
icon: mdi:logout
name: Travel origin
pattern: "[City, Country]"
initial: "City, Country"
input_select:
travel_destination:
icon: mdi:login
name: Travel destination
options:
- Select
- Not yet set
initial: Select
# - zone.home
# - device.tracker.iphone
# - device.tracker.telefoon
travel_origin:
icon: mdi:logout
name: Travel origin
options:
- Select
- Not yet set
initial: Select
# - zone.home
# - device_tracker.iphone
# - device_tracker.telefoon
travel_mode:
icon: mdi:bike
name: Travel mode
options:
- driving
- bicycling
- walking
- transit
initial: bicycling
##########################################################################################
# Automation to set Input_select options with template
##########################################################################################
automation:
- alias: Update Travel Destination options
initial_state: 'on'
trigger:
- platform: state
entity_id: input_text.travel_destination
- platform: homeassistant
event: start
action:
- service: input_select.set_options
data_template:
entity_id: input_select.travel_destination
options:
- Select
- Roosendaal, Netherlands
- Amsterdam, Netherlands
- Eindhoven, Netherlands
- Utrecht, Netherlands
- Antwerp, Belgium
- Brussels, Belgium
- Berlin, Germany
- Bayreuth, Germany
- Paris, France
- '{{ states("input_text.travel_destination") }}'
- alias: Update Travel Destination options
initial_state: 'on'
trigger:
- platform: state
entity_id: input_text.travel_origin
- platform: homeassistant
event: start
action:
- service: input_select.set_options
data_template:
entity_id: input_select.travel_origin
options:
- Select
- Roosendaal, Netherlands
- Amsterdam, Netherlands
- Eindhoven, Netherlands
- Utrecht, Netherlands
- Antwerp, Belgium
- Brussels, Belgium
- Berlin, Germany
- Bayreuth, Germany
- Paris, France
- '{{ states("input_text.travel_origin") }}'
##########################################################################################
# Frontend Group
##########################################################################################
group:
travel_destination:
name: Travel destination
icon: mdi:transit-transfer
entities:
- input_select.travel_origin
- input_text.travel_origin
- input_select.travel_destination
- input_text.travel_destination
- input_select.travel_mode
- sensor.travel_destination
- sensor.distance_selected_destination
- sensor.duration_selected_destination
- sensor.duration_in_traffic_selected_destination