My feeling is that the way Bard rituals are designed is intended to allow them to be cast without being obvious. I'd go so far as to say that is the normal situation with those rituals. If you read most of them they wouldn't make a lot of sense if they had to be obvious. They seem to be of the form "you charm them with your music." That isn't to say that everyone present necessarily has to remain ignorant of the fact that a ritual is being performed. The Arcana skill clearly has use cases defined which would allow a bystander to determine that magic is being employed. Of course in a world where magic is relatively commonplace that might not elicit a significant amount of comment either, bards are after all magically gifted entertainers.
I would say for most other types of rituals the case is less clear. It might be possible to cast them surreptitiously but I think it would require taking steps to do that. Using Bluff would be the obvious technique, but Thievery (legerdemain) could equally well serve. This could be a skill challenge and could easily involve the whole party (some characters distract onlookers, others help block people's view, the caster makes alterations to the casting technique to make it less obvious, etc).
I'd say finally that clever exercise of skills can do a lot of things in terms of bending rules. For instance a ritual might have a casting time of 10 minutes, but perhaps a sufficiently skillful caster could get it done more quickly with a sufficiently good check. The simplest way might be to just require a downward adjustment to the ritual check itself if it has one in return for a 'fast cast'. For rituals without a check one might be imposed to see if the caster is able to figure out how to speed it up without failing. An increased cost could be another option for some cases.
Other potential options exist in cases where the PCs might be able to plan ahead sufficiently. Perhaps a ritual can be 'pre-cast' and embodied within an item for near instant use with sufficient knowledge. This could be something like distilling the essence of the ritual into a potion or a gem or something like that. It would probably be considerably more expensive and might not be as reliable or have the full effect, but it could still be useful. DMs are going to have to look at different rituals and decide what can be allowed and what the trade-offs are to avoid abuse, but any of these things should be reasonable in general and not violate the spirit of the system with thought.
