Go, explore and have fun. Tallinn is great. Small city ... well, at least the area near the old town is ... there's a lot of soviet sprawl outside of that.
There are beaches, but it's not a place you'd think "I'll go to the beach". Very lively in the old town and the bars ... often with drunk Finns. They like the odd ruck as well ... but usually amongst themselves. Old city walls worth a look. There's a market down near the port which used to have a load of "interesting" stuff for sale. I'm guessing the soviet stuff will be cheaper now than it was.
It's a very modern, forward thinking pace ... people were paying for stuff by apps when I was there 20 years ago.
Helsinki ... great city. The green, park type area between the port and town is nice. Loads of nice bars and cafes. It's a rocktastic city if you are into loud guitar music.
It's a bit brutalist in places but worth searching out stuff about Finnish design/architecture: Alvar Aalto, Timo Sarpaneva, Tapio Wirkala etc.
It might be worth considering a circular route via Turku, the archipelago and Stockholm .... which would be a great place to end a journey and fly home .... especially as you will be skint.