I suspect your question isn't really proper for the site because it's looking for opinions across a rather broad range of game activity.

There are a lot of activities in EVE and making an answer that clearly states what cannot be done without an alt could cause further debate.  A player can do most things in EVE (It may simply take an astronomical amount of time).  A player with an alt can do those same things but more effectively if they have the knowledge to do so.  The direct answer to your question is that not having an alt only locks out activities that require you to be in two or more places at once.  So it's more a cost of how much time activities are, rather than being unable to do them at all.

Since your question seems to provoke discussion on what you may be unlikely to do rather than what you cannot do, Reddit may give you better results if you are looking for opinions based on experience.