Do you have a bike already (of any kind)?
Cheapest option is to grab a hamax and try that out if so.
If you need to buy a bike and it's for this purpose, I'd go retro MTB if you want to be cool (means prices have gone up!) or hybrid for cheap and cheerful
yeah the cheapest would be fine, I need to use it for one week only, so.. other options can be fine also.