Mitigate lack of online re-indexing in MS SQL Server

Very small business is running SQL Server 2k5, and will soon upgrade to 2012. They can only afford the Standard version; Enterprise is just too much for them, and they would only use a couple of the features unlocked in the flagship version.

Unfortunately, without Enterprise, they don't get the ability to re-index online tables. They have some large tables (50-100GB) with extremely fragmented indexes (about 45% over 50% fragmented, about 30% over 90% fragmented), and there are noticeable slowdowns. They only have one SQL Server box that can't be taken offline for lengthy maintenance of any sort (read: anything non-critical over 15 minutes). We can take down the smaller and cooler tables for short periods for re-indexing, but the big and hot ones present a problem.

Is there any workaround for this situation?

I noticed that with Win Server 2012 Standard, we get downgrade rights to 2008R2 Enterprise... is there a similar deal for SQL Server? They're on 2005 with a compatibility level of 80, so 2k8 would still be a worthwhile upgrade.