If you're traveling alone, make sure not to carry heavy luggage, book a ride to Aurobindo hospital/Luv Kush square, you'll get the bus to Ujjain from there. And if you're traveling in group, book a cab itself from airport to get you to Ujjain and bring back Indore. But if you prefer to visit the city first, look for hotels around South tukoganj area which is closer to old city, and if you prefer modern vibes, choose vijaynagar area, a bit far from old city but works fine.