The MTR airport express stops at Tsing Yi, Kowloon station and HK station. From HK station you can take the overhead walkway and be at the harbor on the HK island side in ~5mins. You have plenty options from there, you can ride the Ferris wheel, visit the maritime museum, which is nice, take the ferry to the Kowloon side, stroll around HK Central, or walk, the take the bus or the taxi to the historic peak tram, which takes you to Victoria peak with a nice view over the city
https://www.thepeak.com.hk/en/2_1.asp
You can easily do that if you're on time. Yes, the airport express takes less than 30mins. The bus has some scenic views, but you also get some of these from the airport express and it is faster. Unlike most European/US airports, HK airport is very well organized, and if you're already checked in, arriving 45 mins before takeoff is plenty.
Immigration takes like 5mins, no visa with EU passport, don't bother with coins, get an octopus (cashless travel) card at the airport at the octopus station in the arrivals hall, under the airport express banner . Costs 50 HK$ deposit and takes 2mins, you get it and the remaining balance back when you return it. Get cash from the HSBC or BOC ATM at the arrivals hall (good rate, no fees, depending on your bank, probably), never change cash, have fun.