Hi, Rebekah,
Getting to Stirling is very easy by train or bus. There is very frequent service from Edinburgh. Once you get to Stirling, it's about a 20 minute walk to the castle. It's uphill all the way, but the consolation is that it's downhill on the way back. Stirling is a beautiful town, although the central business area is a bit run down.
The Wallace Monument is in Bridge of Allan. It's about 2 1/2 miles from Stirling. You'll be able to see the monument from the battlements of the castle. To get to the monument, you can take the bus from the Goosecroft Road bus station, which is a short walk from the train station, and is just below the Thistles Center, which is about as close to an American mall as you're going to find in Scotland.
The bus service to the Wallace Monument is operated by First Bus. You can take either the 52 bus or the UL bus, which serves Stirling University. Both buses run very frequently, and drop you off near the base of the monument. It's a long climb to the top, but worth it. And again, it's downhill on the way back. Check with your bus driver to find out where the bus stop is for your return journey.
In Falkirk, you'll want to get off the train (if you're taking the train) at Falkirk Grahamston Station. There are two train stations in Falkirk. To get to the Kelpies, walk a couple of blocks to the bus station at the Asda supermarket in Upper Newmarket Street, then catch the First Bus route 2 bus for Borrowstoneness, which will be abbreviated Bo'ness on the bus route display. The bus does not go directly to the Kelpies - you'd need a taxi for that. Have the driver let you off at the closest point to the Kelpies, which I believe is the stadium, and from there it's about a mile to the Kelpies. Many Scots park at the stadium to avoid paying three pounds to park at the Kelpies.
To get to the Falkirk Wheel, return to the Asda and take the number 6 bus for Camelon. Most of the buses go directly to the Wheel. Some don't. You should look for one which goes to Tamfourhill, and the Falkirk Wheel Visitor Centre. The buses run about every half hour. Make sure that you get on the number 6 bus going the correct way. Buses going in the opposite direction will take you to Larbert and Stenhousemuir.
At one point a few years ago, there was a hop on - hop off bus which served both locations in Falkirk, as well as other sites. That service appears to have fallen by the wayside. If anyone has information on that service (route 100), let us know.
I hope that's enough information to get you started. If you have enough time in Falkirk, you should visit Callander House and the Antonine Wall, both of which are within walking distance of the train station.
Slainte!
Mike (Auchterless)