The Standby database can only be opened in ReadOnly mode – if you want to be able to put it BACK into Standby mode. You can do the backup then, or you can shut it down altogether and perform a cold backup if you prefer and if time permits. I still do all my HotStandbys manually, but I’m sure you can either pause the application of logs, or just let them pile up until the backup is done. At which point it will apply them in bulk.
You can also look into Oracle Streams, which I believe is a no cost option. It also does log based replication, but this way the 2nd database would be UP all the time. You might even choose to use it to offload work by using it for reporting or something.
I hope this helps!