there's one of those hop on hop off bus things that's decent as it includes a tram ride up to the castle and takes you out to the Monument to the Discoveries which is worth a look
that said, one of the routes appeared to have nowt on it, but passes the time
as carlos said, once that's done, it's about mooching around and having a beer and that, and the cherry liquer they do
when we were there, there was a festival going on which i think they do throughout june in alfama (i.e. the windy, hilly, cramped, authentic streets), so that's a good month to go for that sort of thing
old town good for boozing
generally just a good relaxed vibe about the place, not too busy like some capitals
easy to get hold of weed