I suppose to everybody the word "reasonable" can amount to a big difference but here are some suggestions.
Mt. Roberts tram. Board right at the dock. No need to prebook. Don't bother if the weather's bad. About $22 each for an all day pass as I recall.
Whale watch. A bit over $100. I felt it was worth it. You will get up close and personal to humpbacks and maybe some orca. Plenty of seals too.
Mendenhall glacier: $10 each round trip from the dock on a school type bus. Another few bucks to enter the visiter center. No need to prebook this trip. Possibly able to be dropped off on the way back from whalewatch trip so you can save $5 each. At the end of the parking lot is a small wooden overlook. At the right time of the year you'll see salmon spawning. It's actually a bear viewing area however we've been 3 times (August) and never seen any bears.
Around town and on the drive to Mendenhall look for bald eagles. If the tide's out you should see plenty on the shore of the channel. They're everywhere. Also Goshawks.
Walk the town if you're able. Somewhat hilly but pretty easy to walk to the russian church and museums. Plenty of shopping as well.
White pass rail. Also a bit over $100. A must if you love trains and/or history. Walk the town. Easy walk from some of the docks. Dock by the graffitti wall has a tram I believe for a small charge.
Minivan tours as well to WhitePass. Not as scenic as the train. Parallels the track for a bit towards town. A nice trip but we preferred the train but the van was quite a bit cheaper.
Cemetary. Quite popular. We were underwhelmed but also plenty of history.
Easy to walk the town up to Creek street. Go up the funicular and walk down the married man's trail. Funicular is about 2 bucks as I recall. At the base of the trail is the salmon ladder. Plenty of salmon to see on our visits in August.
Duck tour was really silly...in fact so silly, it was a lot of fun. Can't remember the cost however but not too much. We prebooked directly but people were buying tickets at the dock.
Lumberjack show is an easy walk from the docks as well. We haven't gone but it's quite popular.
There are many totems in town and at the hotel on the top of the tramway. There are also a couple of totem park tours outside of town.
All towns have tons of shopping.
Enjoy Alaska. It's a terrific place and we can't wait to return.