Ben Goldacre has kept me amused over the years, he's just published an interactive map project (with a team) 'nerdy day trips' and asked for comments, so I thought I'd repay him with some ideas and publish it as a blog rather than just put it in an email.
Look and Layout: I liked the look: Professional look with a quirky feel. Maybe save some map space by thinning out the header or by incorporating the graphic in the top left of the screen into the header? Users generally like having more map, less graphics.
A minor point is that most users will look top left for the zoom bar in any Google Map. You might like to move it up there for them if you don't feel it jinxes your look too much.
User Interface: Nothing major wrong here either. Minor point, I didn't know the post code of the Elan Valley trip I added in Shropshire, I imagine other users won't know postcodes of sites either so it shouldn't be compulsory to add one. Do you really need the user to enter this data at all? You have the latitude/longitude from their point and can generate post codes using this data.
Users will be expecting the info bubble to be graphically linked to the point that was clicked to create it. By not using this you clear more space for your information and therefore less scrolling necessary which is good. However, you might like to think about animating the info screen so that it grows out of the point that created it in some way.
Symbols: A more important point - The paddles you currently have are bigger than necessary. Reduce their size and avoid 'herd of sheep' visual clutter (where paddles obscure other paddles) in more areas. You may like to consider using small circles as this would help even more (you can see this effect by doing a search on Google Maps for "cafes" over London). The problem with this is that you really want to make them an intense red so they are visible and that doesn't fit with your color scheme.
Future features: I'd endorse your feature idea of categories but don't go overboard with very many - a great charm of this map is that it is so simple, don't spoil that by letting it get overly complex.
If I had one suggestion of my own it would be that you work on the day trip creation user interface. I would have it so that a user creates a day trip by dragging a symbol to the right place. When I created my day trip by a simple click I wasn't zoomed in enough on the map to locate it accurately. Dragging and dropping a symbol is not only intuitive, it gets over this issue, people will only do it when they're ready.