I came across this as I was searching for fun things to do at Barcelona. Signed up for their night tour for 2 hours (6.00 - 8.00 pm). Though it was winter and really cold at night (mid-December), my friend and I didn't regret this decision at all. Our English-speaking guide was sick that day so we had a Spanish-speaking guide with us throughout the trip. As we had gone around in Barcelona on a bike and bicycle, we were not too concerned as we were familiar with the sights we were going to visit. It was definitely a fun way of touring part of the city.
If you'd like to try it, I have a few tips for you:
1) If it's your first time touring the city, I'd recommend that you go for this tour in the morning/afternoon. If you've joined another tour (walking or riding) before this, it'd be fun to go for an evening/night tour as you'll get to see a different side of the attractions that you might have visited at night.
2) Ensure that you are able to ride a bike/bicylce or have a sense of balance. You don't need to be a pro at riding though. Initially, you may be scared of it as it's a new vehicle that you're trying to handle. But within 5 to 10 mins of practice and being on it, you will get comfortable and soon enough, you'll be an expert.
3) Be prepared to try something new and enjoy. Remember to take photos at pit-stops.
So, if you'd like to zip around the sights within 2 hours in a fun way, I recommend that you join this tour. You may want to check their Facebook page and be assured that people of all ages can join this activity and will enjoy it. =)