Longyearbyen is a small town, and most of the attractions are within walking distance. However, to get to the Lars Glacier, you need to take a snowmobile ride, cross by boat, and hike. You can book a guided tour that will provide transportation, and the tours usually cost around $150 per person.
If you have more time, you can explore other outdoor activities such as dog-sledding, snowmobiling in the Arctic, cruising to see polar bears, reindeers, and seals. If you have less time, you can prioritize your itinerary with the most interesting attractions that fit your travel preferences.