To Mars: Life
What Mars doesn't have is biological life and the products that come from it.
Soil: I don't care what Mark Whatney says, you can not make large volumes of soil with poop. Martian 'sand' is actually a fine dust, and while hydroponics will get you so far, there will be a time when people will want to plant with rich soil. The first real tree, for example. Soil takes generations to make by itself, with worms and bugs breaking down plant matter, and yes, dead things and poop enriching it.
Even with human intervention, "making" soil to the quantities that Mars needs will be close to insurmoutable. Heaps of soil could come from Earth to get it started, as well as the types of compounds that can help make soil out of Mars' fine dust and regolith.
Petroleum: Even if all of your energy sources come from the sun, you'll still want to make plastics on-site at Mars. Petroleum comes from dead things, as a fossil fuel, that Mars presumably doesn't have.
Life: It's fun to grow your plants animals and population, but it's a lot quicker to import a lot of that.
From Mars: Service Economy & Logistics
Intellectual capital: Mars will need to diversify, because it really doesn't have much going for it. However, there will presumably be a lot of scientific research that can be conducted there, that can't happen on Earth (because of Mars unique atmosphere, etc.), especially just furthering our understanding of living in space: psychology, physiology, and all the sciences related to space life and travel.
Tourism: This should be pretty self explanatory. Today, a honeymoon at the Maldives (or any remote location) could run an American couple ten thousand USD or more. If that's the same cost in the future to get to Mars, I WILL PAY.
Transportation (& Logistics): Mars is a nice enough stepping point to other places in the solar system. The Moon or other places might be better in terms of their makeup, but your question presumes a population of skilled people on Mars. Dubai was about to run out of oil, so they invested in making themselves into a massive hub of trade and transport, some would argue successfully. Mars with limited resources will have to do the same. Be a desert planet that booms into a major center.
Communications: Mars could establish itself as a center for communication between the inner planets and the outer planets, and can also invest on being a focus of arts & entertainment like aslum said in her answer.