Whether you want to explore continental Croatia, i'ts sunny golden fields, green hills, i'ts rivers and lakes. But do not think that the unique landscape is the only advantage of this part of Croatia, there are also medieval castles and fortresses of famous noble families that will take you on a historical journey.
Osijek as your perfect starting point!
For example, the capital city Zagreb is about 300 km away, Budapest in the north is just over 250 km, Novi Sad in the east is about 100 km away, while southern cities such as Sarajevo and Mostar are just over 250 km away, or 400 km.
Reaching these destinations depends entirely on your budget and affinity. Of course, the cheapest thing would be to ask for a ride from your Croatian friend, if you have one. You can also hitchhike, but it can be time consuming and exhausting.
When planning a trip, also consider public transport, such as buses, trains, planes. They are generally a reliable choice and will allow you alternative transportation in case of unforeseen circumstances. You can also rent a car for more freedom, especially if you are traveling in a group.