Or a river... can you build a causeway to divert it? (Admittedly the boys attempt to divert the whole of the Tay round Moncrieffe island was always going to be a heroic failure).
A flow of traffic.... you could always count it!
Or find a view.... just look at it! (The Slivery Tay, from Kinnouill Hill)
Total expenditure, less that £5 as two of the three books were presents!