This was my first time in Milos and it is now definitely up there with my favourite islands, so here are some recommendations...
Klima beach - this was probably my favourite spot, unlike anything I've ever seen before. It's a fisherman's village with colourful houses, lovely locals and the most beautiful sunset. This place really should be at the top of your list for an afternoon activity. There is also a highly rated restaurant there, right on the seaside. Otherwise you can go to various restaurants in Tripiti that overlook the sea from above.
Getting there: You can take the bus to Tripiti and then it is a 30 minute downhill walk through a very scenic route.
Kleftiko - this is a beach that can't be missed, but you can only go there by boat. Unfortunately we only managed to book a half-day boat trip that only went to 2 beaches. However a trip that got recommended to us was Captain Yiangos. This boat tour is from 9am until 7pm, with several stops including a stop at the nearby island Kimolos, for lunch. The whole Captain Yiangos tour is 50 euros.
Sarakiniko beach - aka moon beach - This place is a must see and probably one of the most insta-famous locations is Milos. The waters are not the nicest and 2 days after we visited this location we overheard someone on the bus say they were stung by jellyfish. However, the rocks are beautiful. The bus goes there and the first bus is at 10 am. I think it would be really worth it, if you hire a car or a quad bike to go there for sunrise.
Mantrakia - You have to come here. Another stunning fisherman's village with one restaurant - Medusa. Medusa is highly popular and if you arrive after 7.30 pm, you will likely have to wait for a table. We were lucky because we arrived earlier but we were also two people, so easy to accomodate. Prices are great here, we got a massive tuna steak for 17 euros which is rare to find in Greece. Again, another ideal sunset location.
Firopotamos - Mantrakia can be conveniently combined with Firopotamos which 45 minutes away on foot. Firopotamos is a quiet beach in Milos, ideal for an afternoon swim. We went swimming in Firopotamos before going for dinner at Medusa.
Plaka - Plaka is the chora of Milos, i.e. the centre with all the cute island shops and restaurants. We had dinner at a restaurant called Avli, which has tables on the street as well as in their decorated front garden. A glass of white wine is 5 euros here. I recommend having dinner there and then walking up to the castle in Plaka to watch the most beautiful sunset. Then you can walk around the shops and I particularly recommend the Sand Museum owned by a previous geologist turned artist. He's a super hospitable and interesting person and who knows, maybe you can help him enrich his pallet. After 9.30 there are several bars that play live music in Plaka so you can easily have a full night there.
Again, like in Sifnos I recommend staying in Adamas (AKA Adamantas) which is the Milos port. The port is also a good time at night with many restaurants and rooftop bars. A lot of boat tours leave from the port or have transfer from the port. Moreover, on the day of your departure you can also just check out and chill at one of the cafes while you wait for your ferry, making for a stress-free travel experience.
Snacks - Again, for snacks you can hit up local bakeries and supermarkets for fresh summer fruit (eg nectarines ;) )and Diana was obsessed with the peanut butter and banana yogurt pot from Grigoris which is conveniently located next to the port bus stop.
Transport - all buses leave from the port and go to many places around the islands. Milos is huge and if you can, its definitely worth it to hire a car. However, like in Sifnos you can easily look up the Milos bus timetable and download the route to Google maps. I recommend looking up the places you want to go and finding the nearest convenient bus stop. If you are up for walking, it is definitely doable. Also people are very nice and will stop for hitchhikers. We hitchhiked twice and it was great. However needless to say we were 2 people which makes it easier to hitchhike than a group of 6. Also if you are hitchhiking, stay safe. I like to only get into cars that have at least one woman.
Milos is a must-see and although we managed to squeeze a lot in the 2 full days (3 nights) we were there and it was super fulfilling as we got to see most places, we were exhausted. Were I to visit again, I would probably stay 5 nights and hire a car.