Why not try an online banking solution ... you would need to have a database (MySQL, free, easy and downloadableif you dont have a DBMS already) with some account details - then write :
1)A database server engine on localhost ports to recieve requests from a webserver, and processes SQL transactions.
2)A webserver that listens on an IP/port for incoming data requests, and then parses/validates those requests, and then passes those requests onto the database server.
3)A webpage/jsp that has a form which then connects to your webserver and passes the data requests.
If you write all of this too quickly, then you could include some heavy duty webserver security gui which allows you to change access restrictions on the fly...
I've been writing a similar project, am most of the way there, and have been doing it part-time for a couple of weeks.
Ben