Hmmm...good question! Monday vs Friday. I'm not quite sure. Both fall under an extended weekend so are busier than Tuesday through Thursday. This past Friday and Monday both seemed pretty busy, but it did barely open. Maybe we should monitor tomorrow and next Monday to decide? Watch the wait times for each day? Are you thinking April 25 or May 2? If May 2 is the possible Monday I'd say go with Friday instead since the beginning of May is a holiday for many people and might be more crowded?
As for tips, I'd say get your Butterbeer from the Hogs Head Pub instead of the carts. The carts always have long lines and I never notice too much congestion at the bar. You want to be able to enjoy your Butterbeer while you wait in line for everything else. Most people tend to prefer frozen Butterbeer because the blend seems more balanced but it gives me brain freeze. I'd say do Ollivander's and the Flight of the Hippogriff first because those lines can get pretty long. FotH is a super short ride so it's not worth waiting too long for. Even if there is a bit of a line to see Forbidden Journey later it'll still be worth it. Plus, there is so much to see and appreciate it that you don't really want to rush through the queue either.
For the interactive wands if you search in the window you can find the hidden sensor. Make sure you aim for that when you cast your spell and the magic will work better. All the food I've had so far at Three Broomsticks has been great so I can't recommend anything over anything else. There are secret light/shadow animations projected on the walls inside at random times that are fun to notice. Dishes flying in one corner, an owl at another. I haven't seen them all yet. If you eat outside though, you get a nice view of Hogwarts.
And my personal favorite secret, mainly because it's my doing, but if you do end up outside of Hogsmeade, go to the second floor of Krusty Burger in Springfield to the back room where the walls are covered in caricatures. There's a drawing of a Simpsonized Severus Snape from the episode when Benedict Cumberbatch impersonated Alan Rickman. There's also a drawing of Daniel Radcliffe as Edmund the Vampire from one of the Treehouse of Horror episodes but that is harder to recognize. The asked us on the Simpsons crew to submit artwork for those walls and I had to include Harry Potter references and Universal used them!
Hope that helps! Enjoy your trip!