Aalto University main campus, Otaniemi, is located in Espoo. The closest airport is Helsinki/Vantaa, which provides direct flights to/from almost 150 destinations.
A train will take you from the airport to Helsinki city in about 30 minutes (5.50€), and a taxi in about 30min to Helsinki or Otaniemi (~40€). There is a new metro connection between Helsinki city and Otaniemi (15min, 5.50€). If you choose to stay in Helsinki, we recommend buying a 2-day regional ticket (21€/48h) right at the airport: ticket machines are located in the baggage halls 2A and 2B as well as in several other locations.
Finnish hotels are typically safe, functional, and clean, even those with less stars. By default, a breakfast buffet is included in the room price.
If you fancy a more vibrant environment, Helsinki city center provides a wide variety of accommodation options. Hotels nearby the Helsinki railway station might be the handiest ones for moving to the airport and Otaniemi, check e.g. Holiday Inn Helsinki City Center or Hotel Vaakuna. If you are into automation and do not care about breakfast at the hotel, an unmanned Omena hotel is an interesting option to consider!