Santorini has plenty of stuff you can do without spending much Money. As I only stayed there for three days, I tried to do everything I could!
1. Watch the sunset in Oya
This is the most popular thing to do in Santorini and it's completely worth it. The sunset in Oya is, indeed, amazing. But be sure to be there at least one hour before the sunset, because everyone gathers in the little streets to see it. Some people even climb walls and houses to have a better view and take nice pictures, it's crazy! Also, it'll take you about half na hour to be able to walk out of the crow after the sun sets.
2. Drink some greek wine... with a view
Santorini is famous for its wines, which are also really affordable, and is filled with many winery shops along the Island. We went to Santo Wines, which had a breathtaking view. You have the option to wine taste, or can simply drink a glass of wine while enjoying an amazing landscape. I payed 1,5€ for my glass, as far as I remember.
3. Go to the red sand beach
This is the only beach in Europe which has red sand. It's pretty impressive and unique, but be prepared for a thrilling walk down until you reach the sea shore. You can also take amazing photos there and, needless to say, the water is warm.
4. Visit Thira at night
Thira is beautiful not only during the day, but also at night with its cute lights and vivid nightlife. You might want to have a drink at a pub or eat in a traditional greek food restaurante. Oya is beautiful at night too, but there's not much to there after nine o'clock. So, going to Thira might be the best option.
5. Ride a donkey
This is for the adventurous ones! Donkeys are very popular in Santorini because it's a good means of transport to go up and down the cliffs. For 5€ you can ride a donkey, going up or down the cliff. I'm warning you: it's scary! Because donkeys are big and going down can seem pretty dangerous. However, I don't think you shouldn't miss this experience.
6. Buy donkey milk lotion and natural sponges
Donkey milk lotion is insane, it hydrates the skin so much! In Santorini you can find it anywhere and it's a good souvenir. Natural sponges are also very popular in this Island. They feel so soft on the skin, you should definetely try them.