The majority of transportation is in the form of minibuses (see below). Ask locals where to get the specific bus you are looking for though because there are a couple points in town where they gather and not every pick up point will have a bus servicing every town.
I stayed at Sinar Harapan while in Probolinggo and HIGHLY recommend it. Call the number on the sign in the photo below and ask them to pick you up when you get to town. Not only will you be travelling in style via their baby blue Beetle but, its also complimentary. The rooms are a great price, the place is well maintained and the owner is extremely knowledgable about wherever you want to go. Use him for information only, try not to get sucked into a tour package. You will have a much bigger adventure and more freedom if you travel on your own schedule and by your own means.
Wikitravel: I've attempted to use wikitravel for a couple different trips without much satisfaction. Surprisingly for it was a huge help to me in Indonesia.
Be My Travel Muse: This is a blog by another solo female traveller. The post for Bromo was amazingly helpful and gave great, clear and simple, instructions for seeing the crater without a tour. Check it out!
I think its very important to be an informed traveller and this is one location where I would very strongly advocate you research before you visit. The sulphur mining industry that offers this tourist attraction to us is also an extremely dangerous and exploitative one. The men working tirelessly are making 50,000 to 75,000 Rupiahs a day (less than 10$). While here I saw tourists force workers off walking paths and treat them like things to be photographed rather than people with a difficult job to do. Don't fall into the role of ignorant traveller, read up a bit before you visit.
With that out of the way, back to travel talk. I always travel with budget at the forefront of my mind and this trip was no exception. I was determined not to pay anymore than necessary and that was no easy task. Many people choose to visit Ijen as part of a tour from Bondowoso or even Probolinggo. Those who choose to visit on their own are few and far between but often end up with a tour anyways. I stayed at Arabika Coffee Plantation, one of two popular coffee plantations offering accommodation in the area. From the moment I arrived to the moment I left I was hassled and pressured into getting a tour guide for the morning. I opted to pay only for transportation though and hired a mototaxi driver to pick me up at 1:00am. The ride to the ticket gate was quite long and there is an entrance fee which you must pay. However, beyond that, visiting Ijen on your own is simple and worth it. The climb up to the sulphur crater is 3km of steep uphill in the blackness of early morning and then an even steeper and shale ridden descent into the crater.. I'd suggest bringing a headlamp to keep yourself safe. Enjoy the photos and again, some more information can be found by hovering your mouse over the photos.
Borobudur, Candi Prambanan and Kopi Luwak
The tour started with a visit to Borobudur. This monument is the largest Buddhist temple in the world and was built in the 9th century by Mahayana Buddhists. The monument is made up of 9 levels with 2,672 relief panels and 504 Buddha statues in various iconic poses. At the top of the monument are 72 Buddhas set inside stupas. It is an extremely impressive sight and with breath-taking views of the valleys surrounding it.
Next, the tour went to Candi Prambanan, a 9th century Hindu temple dedicated to the three central gods of Hinduism, Brahma (the creator), Vishnu (the preserver), and Shiva (the destroyer). This is the largest Hindu temple in Indonesia.
Finally, the tour made a stop at a gift shop specializing in silver items and kopi luwak coffee. This coffee is one of the most expensive in the world and attributes its unique taste to its passage through the digestive tract of civet cats after the small animal eats the ripe coffee berry. I did take the opportunity to taste this famous brew, however, drinker beware, major controversy surrounds the captivity and force-feeding of civets. In my photo above you may note the civet living at this particular shop had very little room to move around and was being fed a diet made almost solely of coffee berries. Kopi Luwak coffee is sold in the US for between $100 and $600 a pound. Is it worth it? In my opinion it just tasted like coffee. I had my first and last cup of kopi luwak on the tour and am satisfied.