Difficult to not bring one if you're not staying over really. You'd need chargers, passports, sun cream, money, water etc. Small bag it is and stuff everything into it!
Here's a life hack for you;
Find any 5 star hotel, say you've checked out in the morning and ask if you can drop your bag there for them to keep
Go and collect it later.
I've done it countless times at the St Pancras Hotel in London, and others on day trips
Just be confident. It's unlikely that they will quiz you, 5 star hotels do whatever they can to make customer experience smooth as possible